Revision as of 05:54, 3 June 2009

Introduction

ATL, the Atlas Transformation Language, is the ATLAS INRIA & LINA research group's answer to the OMG MOF/QVT RFP. It is a model transformation language specified as both a metamodel and a textual concrete syntax. In the field of Model-Driven Engineering (MDE), ATL provides developers with a mean to specify the way to produce a number of target models from a set of source models.

The ATL language is a hybrid of declarative and imperative programming. The preferred style of transformation writing is the declarative one: it enables to simply express mappings between the source and target model elements. However, ATL also provides imperative constructs in order to ease the specification of mappings that can hardy be expressed declaratively.

An ATL transformation program is composed of rules that define how source model elements are matched and navigated to create and initialize the elements of the target models. Besides basic model transformations, ATL defines an additional model querying facility that enables to specify requests onto models. ATL also allows code factorization through the definition of ATL libraries.

Developed over the Eclipse platform, the ATL Integrated Development Environment (IDE) provides a number of standard development tools (syntax highlighting, debugger, etc.) that aim to ease the design of ATL transformations. The ATL development environment also offers a number of additional facilities dedicated to models and metamodels handling. These features include a simple textual notation dedicated to the specification of metamodels, but also a number of standard bridges between common textual syntaxes and their corresponding model representations.

Installation

Prerequisites

Java

Download and install a JVM (JRE or JDK), if one is not already installed.
- for ATL 2.0 or older, JDK 1.4 will work.
- for ATL 3.0+, you need JDK 5.0 or later

Eclipse

Download and install Eclipse.

EMF

Install EMF.

Install ATL

There are two ways to install ATL:

via update site
via zip: see ATL Downloads. Once your zip is downloaded, just unpack it into
- with Eclipse 3.3, 3.2: ~/eclipse/features/ and ~/eclipse/plugins/ folders
- with Eclipse 3.4+: ~/eclipse/dropins/ folder

Finally, restart Eclipse.

The ATL SDK build enables to:

consult documentation offline
access ATL examples
access to the source code of installed ATL plugins

To install ATL from CVS, please refer to the Developer Guide.

Overview of the Atlas Transformation Language

The ATL language offers ATL developers to design different kinds of ATL units. An ATL unit, whatever its type, is defined in its own distinct ATL file. ATL files are characterized by the .atl extension.

As an answer to the OMG MOF/QVT RFP, ATL mainly focus on the model to model transformations. Such model operations can be specified by means of ATL modules. Besides modules, the ATL transformation language also enables developers to create model to primitive data type programs. These units are called ATL queries. The aim of a query is to compute a primitive value, such as a string or an integer, from source models. Finally, the ATL language also offers the possibility to develop independent ATL libraries that can be imported from the different types of ATL units, including libraries themselves. This provides a convenient way to factorize ATL code that is used in multiple ATL units. Note that the three ATL unit kinds same the share .atl extension.

These different ATL units are detailed in the following subsections. This section explains what each kind of unit should be used for, and provides an overview of the content of these different units.

Examples metamodels

This section provides two simple metamodels which will be used all along this guide to demonstrate ATL syntax and use.

Author metamodel

Person metamodel

Biblio metamodel

ATL module

An ATL module corresponds to a model to model transformation. This kind of ATL unit enables ATL developers to specify the way to produce a set of target models from a set of source models. Both source and target models of an ATL module must be "typed" by their respective metamodels. Moreover, an ATL module accepts a fixed number of models as input, and returns a fixed number of target models. As a consequence, an ATL module can not generate an unknown number of similar target models (e.g. models that conform to a same metamodel).

Structure of an ATL module

An ATL module defines a model to model transformation. It is composed of the following elements:

A header section that defines some attributes that are relative to the transformation module;
An optional import section that enables to import some existing ATL libraries;
A set of helpers that can be viewed as an ATL equivalent to Java methods;
A set of rules that defines the way target models are generated from source ones.

Helpers and rules do not belong to specific sections in an ATL transformation. They may be declared in any order with respect to certain conditions (see ATL Helpers section for further details). These four distinct element types are now detailed in the following subsections.

Header section

The header section defines the name of the transformation module and the name of the variables corresponding to the source and target models. It also encodes the execution mode of the module. The syntax for the header section is defined as follows:

module module_name;
create output_models [from|refining] input_models;

The keyword module introduces the name of the module. Note that the name of the ATL file containing the code of the module has to correspond to the name of this module. For instance, a ModelA2ModelB transformation module has to be defined into the ModelA2ModelB.atl file. The target models declaration is introduced by the create keyword, whereas the source models are introduced either by the keyword from (in normal mode) or refining (in case of refining transformation). The declaration of a model, either a source input or a target one, must conform the scheme model_name : metamodel_name. It is possible to declare more than one input or output model by simply separating the declared models by a coma. Note that the name of the declared models will be used to identity them. As a consequence, each declared model name has to be unique within the set of declared models (both input and output ones). The following ATL source code represents the header of the Book2Publication.atl file, e.g. the ATL header for the transformation from the Book metamodel to the Publication metamodel:

module Book2Publication;
create OUT : Publication from IN : Book;

Import section

The optional import section enables to declare which ATL libraries have to be imported. The declaration of an ATL library is achieved as follows:

uses extensionless_library_file_name;

For instance, to import the strings library, one would write:

uses strings;

Note that it is possible to declare several distinct libraries by using several successive uses instructions.

Helpers

ATL helpers can be viewed as the ATL equivalent to Java methods. They make it possible to define factorized ATL code that can be called from different points of an ATL transformation. An ATL helper is defined by the following elements:

a name (which corresponds to the name of the method);
a context type. The context type defines the context in which this attribute is defined (in the same way a method is defined in the context of given class in object-programming);
a return value type. Note that, in ATL, each helper must have a return value;
an ATL expression that represents the code of the ATL helper;
an optional set of parameters, in which a parameter is identified by a couple (parameter name, parameter type).

As an example, it is possible to consider a helper that returns the maximum of two integer values: the contextual integer and an additional integer value which is passed as parameter. The declaration of such a helper will look like (detail of the helper code is not interesting at this stage, please refer to ATL Helpers section for further details):

helper context Integer def : max(x : Integer) : Integer = ...;

It is also possible to declare a helper that accepts no parameter. This is, for instance, the case for a helper that just multiplies an integer value by two:

helper context Integer def : double() : Integer = self * 2;

In some cases, it may be interesting to be able to declare an ATL helper without any particular context. This is not possible in ATL since each helper must be associated with a given context. However, the ATL language allows ATL developers to declare helpers within a default context (which corresponds to the ATL module). This is achieved by simply omitting the context part of the helper definition. It is possible, by this mean, to provide a new version of the max helper defined above:

helper def : max(x1 : Integer, x2 : Integer) : Integer = ...;

Note that several helpers may have the same name in a single transformation. However, helpers with a same name must have distinct signatures to be distinguishable by the ATL engine (see ATL Helpers section for further details). The ATL language also makes it possible to define attributes. An attribute helper is a specific kind of helper that accepts no parameters, and that is defined either in the context of the ATL module or of a model element. In the remaining of the present document, the term attribute will be specifically used to refer to attribute helpers, whereas the generic term of helper will refer to a functional helper. Thus, the attribute version of the double helper defined above will be declared as follows:

helper context Integer def : double : Integer = self * 2;

Declaring a functional helper with no parameter or an attribute may appear to be equivalent. It is therefore equivalent from a functional point of view. However, there exists a significant difference between these two approaches when considering the execution semantics. Indeed, compared to the result of a functional helper which is calculated each time the helper is called, the return value of an ATL attribute is computed only once when the value is required for the first time. As a consequence, declaring an ATL attribute is more efficient than defining an ATL helper that will be executed as many times as it is called. Note that the ATL attributes that are defined in the context of the ATL module are initialized (during the initialization phase) in the order they have been declared in the ATL file. This implies that the order of declaration of this kind of attribute is of some importance: an attribute defined in the context of the ATL module has to be declared after the other ATL module attributes it depends on for its initialization. A wrong order in the declaration of the ATL module attributes will raise an error during the initialization phase of the ATL program execution.

Rules

In ATL, there exist three different kinds of rules that correspond to the two different programming modes provided by ATL (e.g. declarative and imperative programming): the matched rules (declarative programming), the lazy rules, and the called rules (imperative programming).

Matched rules. The matched rules constitute the core of an ATL declarative transformation since they make it possible to specify:

1) for which kinds of source elements target elements must be generated,

2) the way the generated target elements have to be initialized.

A matched rule is identified by its name. It matches a given type of source model element, and generates one or more kinds of target model elements. The rule specifies the way generated target model elements must be initialized from each matched source model element. A matched rule is introduced by the keyword rule. It is composed of two mandatory (the source and the target patterns) and two optional (the local variables and the imperative) sections. When defined, the local variable section is introduced by the keyword using. It enables to locally declare and initialize a number of local variables (that will only be visible in the scope of the current rule). The source pattern of a matched rule is defined after the keyword from. It enables to specify a model element variable that corresponds to the type of source elements the rule has to match. This type corresponds to an entity of a source metamodel of the transformation. This means that the rule will generate target elements for each source model element that conforms to this matching type. In many cases, the developer will be interested in matching only a subset of the source elements that conform to the matching type. This is simply achieved by specifying an optional condition (expressed as an ATL expression, see OCL Declarative Expressions section for further details) within the rule source pattern. By this mean, the rule will only generate target elements for the source model elements that both conform to the matching type and verify the specified condition.

The target pattern of a matched rule is introduced by the keyword to. It aims to specify the elements to be generated when the source pattern of the rule is matched, and how these generated elements are initialized. Thus, the target pattern of a matched rule specifies a distinct target pattern element for each target model element the rule has to generate when its source pattern is matched. A target pattern element corresponds to a model element variable declaration associated with its corresponding set of initialization bindings. This model element variable declaration has to correspond to an entity of the target metamodels of the transformation.

Finally, the optional imperative section, introduced by the keyword do, makes it possible to specify some imperative code that will be executed after the initialization of the target elements generated by the rule. As an example, consider the following simple ATL matched rule between two metamodels, MMAuthor and MMPerson:

rule Author {
	from
		a : MMAuthor!Author
	to
		p : MMPerson!Person (
			name <- a.name,
			surname <- a.surname
		)
}

This rule, called Author, aims to transform Author source model elements (from the MMAuthor source model) to Person target model elements in the MMPerson target model. This rule only contains the mandatory source and target patterns. The source pattern defines no filter, which means that all Author classes of the source MMAuthor model will be matched by the rule. The rule target pattern contains a single simple target pattern element (called p). This target pattern element aims to allocate a Person class of the MMPerson target model for each source model element matched by the source pattern. The features of the generated model element are initialized with the corresponding features of the matched source model element. Note that a source model element of an ATL transformation should not be matched by more than one ATL matched rule. This implies the source pattern of matched rules to be designed carefully in order to respect this constraint. Moreover, an ATL matched rule can not generate ATL primitive type values.

Lazy rules.

TODO: write lazy rules documentation overview

Called rules. The called rules provide ATL developers with convenient imperative programming facilities. Called rules can be seen as a particular type of helpers: they have to be explicitly called to be executed and they can accept parameters. However, as opposed to helpers, called rules can generate target model elements as matched rules do. A called rule has to be called from an imperative code section, either from a match rule or another called rule.

As a matched rule, a called rule is introduced by the keyword rule. As matched rules, called rules may include an optional local variables section. However, since it does not have to match source model elements, a called rule does not include a source pattern. Moreover, its target pattern, which makes it possible to generate target model elements, is also optional. Note that, since the called rule does not match any source model element, the initialization of the target model elements that are generated by the target pattern has to be based on a combination of local variables, parameters and module attributes. The target pattern of a called rule is defined in the same way the target pattern of a matched rule is. It is also introduced by the keyword to. A called rule can also have an imperative section, which is similar to the ones that can be defined within matched rules. Note that this imperative code section is not mandatory: it is possible to specify a called rule that only contains either a target pattern section or an imperative code section. In order to illustrate the called rule structure, consider the following simple example:

rule NewPerson (na: String, s_na: String) {
	to
		p : MMPerson!Person (
			name <- na
		)
	do {
		p.surname <- s_na
	}
}

This called rule, named NewPerson, aims to generate Person target model elements. The rule accepts two parameters that correspond to the name and the surname of the Person model element that will be created by the rule execution. The rule has both a target pattern (called p) and an imperative code section. The target pattern allocates a Person class each time the rule is called, and initializes the name attribute of the allocated model element. The imperative code section is executed after the initialization of the allocated element (see Default mode execution semantics section for further details on execution semantics). In this example, the imperative code sets the surname attribute of the generated Person model element to the value of the parameter s_na.

Module execution modes

The ATL execution engine defines two different execution modes for ATL modules. With the default execution mode, the ATL developer has to explicitly specify the way target model elements must be generated from source model elements. In this scope, the design of a transformation which aims to copy its source model with only a few modifications may prove to be very tiresome. Designing this transformation in default execution mode therefore requires the developer to specify the rules that will generate the modified model elements, but also all the rules that will only copy, without any modification, source to target model elements. The refining execution mode has been designed for this kind of situation: it enables ATL developers to only specify the modifications that have to be performed between the transformation source and target models. These two execution modes are described in the following subsections.

Normal execution mode

The normal execution mode is the ATL module default execution mode. It is associated with the keyword from in the module header. In default execution mode, the ATL developer has to specify, either by matched or called rules, the way to generate each of the expected target model elements. This execution mode suits to most ATL transformations where target models differ from the source ones.

Refining execution mode

The refining execution mode has been introduced to ease the programming of refining transformations between similar source and target models. With the refining mode, ATL developers can focus on the ATL code dedicated to the generation of modified target elements. Other model elements (e.g. those that remain unchanged between the source and the target model) are implicitly copied from the source to the target model by the ATL engine. The refining mode is associated with the keyword refining in the header of the ATL module. Granularity of the refining mode is defined at the model element level. This means that the developer will have to specify how to generate a model element as soon as the transformation modifies one of its features (either an attribute or a reference). On the other side, the developer is not required to specify the ATL code that corresponds to the copy of unchanged model elements. This feature may result in important saving of ATL code, which, in the end, makes the programming of refining ATL transformations simpler and easier. At current time, the refining mode can only be used to transform a single source model into a single target model. Both source and target models must conform to the same metamodel.

Module execution semantics

This section introduces the basics of the ATL execution semantics. Although designing ATL transformations does not require any particular knowledge on the ATL execution semantics, understanding the way an ATL transformation is processed by the ATL engine can prove to be helpful in certain cases (in particular, when debugging a transformation).

The semantics of the two available ATL execution modes, the normal and the refining modes, are introduced in the following subsections.

Default mode execution semantics

The execution of an ATL module is organized into three successive phases:

a module initialization phase,
a matching phase of the source model elements,
a target model elements initialization phase.

The module initialization step corresponds to the first phase of the execution of an ATL module. In this phase, the attributes defined in the context of the transformation module are initialized. Note that the initialization of these module attributes may make use of attributes that are defined in the context of source model elements. This implies these new attributes to be also initialized during the module initialization phase. If an entry point called rule has been defined in the scope of the ATL module, the code of this rule (including target model elements generation) is executed after the initialization of the ATL module attributes.

During the source model elements matching phase, the matching condition of the declared matched rules are tested with the model elements of the module source models. When the matching condition of a matched rule is fulfilled, the ATL engine allocates the set of target model elements that correspond to the target pattern elements declared in the rule. Note that, at this stage, the target model elements are simply allocated: they are initialized during the target model elements initialization phase.

The last phase of the execution of an ATL module corresponds to the initialization of the target model elements that have been generated during the previous step. At this stage, each allocated target model element is initialized by executing the code of the bindings that are associated with the target pattern element the element comes from. Note that this phase allows invocations of the resolveTemp() operation that is defined in the context of the ATL module. The imperative code section that can be specified in the scope of a matched rule is executed once the rule initialization step has completed. This imperative code can trigger the execution of some of the called rules that have been defined in the scope of the ATL module.

Refining mode execution semantics

TODO: update with the current refining mode execution semantics

ATL Query

An ATL query consists in a model to primitive type value transformation. An ATL query can be viewed as an operation that computes a primitive value from a set of source models. The most common use of ATL queries is the generation of a textual output (encoded into a string value) from a set of source models. However, ATL queries are not limited to the computation of string values and can also return a numerical or a boolean value.

The following subsections respectively describe the structure and the execution semantics of an ATL query.

Structure of an ATL query

After an optional import section, an ATL query must define a query instantiation. A query instantiation is introduced by the keyword query and specifies the way its result must be computed by means of an ATL expression:

query query_name = exp;

Beside the query instantiation, an ATL query may include a number of helper or attribute definitions. Note that, although an ATL query is not strictly a module, it defines its own kind of default module context. It is therefore possible, for ATL developers, to declare helpers and attributes defined in the context of the module in the scope of an ATL query.

Query execution semantics

As an ATL module, the execution of an ATL query is organized in several successive phases. The first phase is the initialization phase. It corresponds to the initialization phase of the ATL modules and is dedicated to the initialization of the attributes that are defined in the context of the ATL module.

The second phase of the execution of an ATL query is the computation phase. During this phase, the return value of the query is calculated by executing the declarative code of the query element of the ATL query. Note that the helpers that have been defined within the query file can be called at both the initialization and the computation phases.

ATL Library

The last type of ATL unit is the ATL library. Developing an ATL library enables to define a set of ATL helpers that can be called from different ATL units (modules, but also queries and libraries).

As the other kinds of ATL units, an ATL library can include an optional import section. Besides this import section, an ATL library defines a number of ATL helpers that will be made available in the ATL units that will import the library.

Compared to an ATL module, there exists no default module element for ATL libraries. As a consequence, it is impossible, in libraries, to declare helpers that are defined in the default context of the module. This means that all the helpers defined within an ATL library must be explicitly associated with a given context.

Compared to both modules and queries, an ATL library cannot be executed independently. This currently means that a library is not associated with any initialization step at execution time (as described in Module execution semantics). Due to this lack of initialization step, attribute helpers cannot be defined within an ATL library.

The ATL Language

This section is dedicated to the description of the ATL language. As introduced in Overview of the Atlas Transformation Language, the language enables to define three kinds of ATL units: the ATL transformation modules, the ATL queries and the ATL libraries. According to their type, these different kinds of units may be composed of a combination of ATL helpers, attributes, matched and called rules. This section aims to detail the syntax of these different ATL elements. For this purpose, the ATL language is based on OMG OCL (Object Constraint Language) norm for both its data types and its declarative expressions. There exist a few differences between the OCL definition and the current ATL implementation. They will be specified in this section by specific remarks.

Data types

The ATL data type scheme is very close, but not similar, to the one defined by OCL. The following schema provides an overview of the data type's structure considered in ATL. The different data types presented in this schema represent the possible instances of the OclType class.

The root element of the OclType instances structure is the abstract OclAny type, from which all other considered types directly or indirectly inherit. ATL considers six main kinds of data types: the primitive data types, the collection data types, the tuple type, the map type, the enumeration type and the model element type. Note that the map data type is implemented by ATL as an additional facility, but does not appear in the OCL specification.

The class OclType can be considered as the definition of a type in the scope of the ATL language. The different elements appearing in the schema represent the type instances that are defined by OCL (except the map and the ATL module data types), and implemented within the ATL engine.

The OCL primitive data types correspond to the basic data types of the language (the string, boolean and numerical types). The set of collection types introduced by OCL provides ATL developers with different semantics for the handling of collections of elements. Additional data types include the enumerations, a tuple and a mapping data type and the model element data type. This last corresponds to the type of the entities that may be declared within the models handled by the ATL engine. Finally, the ATL module data type, which is specific to the ATL language, is associated with the running ATL units (either modules or queries).

Before going further in the description of these data types, it must be noted that each OCL expression, including the operations associated with each kind of data type (that are presented along with their respective data type), is defined in the context of an instance of a specific type. In ATL as in OCL, the reserved keyword self is used to refer to this contextual instance.

OclType operations

The class OclType corresponds to the definition of the type instances specified by OCL. It is associated with a specific OCL operation: allInstances(). This operation, which accepts no parameter, returns a set containing all the currently existing instances of the type self.

The ATL implementation provides an additional operation that enables to get all the instances of a given type that belong to a given metamodel. Thus, the allInstancesFrom(metamodel : String) operation returns a set containing the instances of type self that are defined within the model namely identified by metamodel.

OclAny operations

This section describes a set of operations that are common to all existing data types. The syntax used to call an operation from a variable in ATL follows the classical dot notation:

self.operation_name(parameters)

ATL currently provides support for the following OCL-defined operations:

comparison operators: =, <>;
oclIsUndefined() returns a boolean value stating whether self is undefined;
oclIsKindOf(t : oclType) returns a boolean value stating whether self is an either an instance of t or of one of its subtypes;
oclIsTypeOf(t : oclType) returns a boolean value stating whether self is an instance of t.

The operations oclIsNew() and oclAsType() defined by OCL are currently not supported by the ATL engine. ATL however implements a number of additional operations:

toString() returns a string representation of self. Note that the operation may return irrelevant string values for a few remaining types;
oclType() returns the oclType of self;
asSequence(), asSet(), asBag() respectively return a sequence, a set or a bag containing self. These operations are redefined for the collection types;
output(s : String) writes the string s to the Eclipse console. Since the operation has no return value, it shall only be used in ATL imperative blocks;
debug(s : String) returns the self value and writes the "s : self_value" string to the eclipse console;
refSetValue(name : String, val : oclAny) is a reflective operation that enables to set the self feature identified by name to value val. It returns self;
refGetValue(name : String) is a reflective operation that returns the value of the self feature identified by name;
refImmediateComposite() is a reflective operation that returns the immediate composite (e.g. the immediate container) of self;
refInvokeOperation(opName : String, args : Sequence) is a reflective operation that enables to invoke the self operation named opName with the sequence of parameter contained by args.

The ATL Module data type

The ATL Module data type is specific to the ATL language. This internal data type aims to represent the ATL unit (either a module or a query) that is currently run by the ATL engine. There exists a single instance of this data type, and developers can refer to it (in their ATL code) using the variable thisModule. The thisModule variable makes it possible to access the helpers and the attributes that have been declared in the context of the ATL module.

The ATL Module data type also provides the resolveTemp operation. This specific operation makes it possible to point, from an ATL rule, to any of the target model elements (including non-default ones) that will be generated from a given source model element by an ATL matched rule.

The operation resolveTemp has the following declaration:

resolveTemp(var, target_pattern_name)

The parameter var corresponds to an ATL variable that contains the source model element from which the searched target model element is produced. The parameter target_pattern_name is a string value that encodes the name of the target pattern element that maps the provided source model element (contained by var) into the searched target model element.

Note that, as it is defined in the scope of the ATL module, this operation must be called from the variable thisModule. The resolveTemp operation must not be called before the completion of the matching phase. This means that the operation can be called from:

the target pattern and do sections of any matched rule;
the target pattern and do sections of a called rule, provided that this called rule is executed after the matching phase (e.g. is not called from a transformation entrypoint).

ATL developers may note that the operation call does not specify the matched rule from which the generated target model element comes from. However, as explained in the Rules section, a source model element should not be matched by more than one matched rule. As a consequence, the concerned matched rule can be derived from the specified source model element.

Primitive data types

OCL defines four basic primitive data types:

the Boolean data type, for which possible values are true or false;
the Integer data type which is associated with the integer numerical values (1, -5, 2, 34, 26524, ...);
the Real data type which is associated with the floating numerical values (1.5, 3.14, ...);
the String data type ('To be or not to be', ...). A string is defined between '. The escape character '\' enables to include ' characters within handled string variables. Note that, in OCL:
- a character is encoded as a one-character string;
- the characters composing a string are numbered from 1 to the size of the string.

According to the considered data type (string, numerical values and boolean values), OCL defines a number of specific operations. They are detailed in the following sections along with some additional functions provided by the ATL engine.

Boolean data type operations

The set of OCL operations defined for the boolean data type is the following:

logical operators: and, or, xor, not;
implies(b : Boolean) returns false if self is true and b is false, and

returns true otherwise.

Boolean expressions evaluation

In this case:

if (exp1 and exp2)
then ...
else ...
endif

exp2 will always be evaluated, regardless of the result of the first expression. ATL evaluates it like this:

if (exp1.and(exp2))
then ...
else ...
endif

So remember that in this case:

if (self.attributes->size() > 0
     and self.attributes->first().attr)

Even though the first member is false, there may be a call of the "attr" property on an undefined element, which will cause an error.

String data type operations

OCL defines the following operations for the string data type:

size() returns the number of characters contained by the string self;
concat(s : String) returns a string in which the specified string s is concatenated to the end of self;
substring(lower : Integer, upper : Integer) returns the substring of self starting from character lower to character upper;
toInteger() and toReal().

Besides the OCL-defined operations, ATL implements a number of additional operations for the string data type:

comparison operators: <, >, =>, =<;
the string concatenation operator (+) can be used as a shortcut for the string concat() function;
toUpper(), toLower() respectively return an upper/lower case copy of self;
toSequence() returns the sequence of characters (e.g. of one-character strings) corresponding to self;
trim() returns a copy of self with leading and trailing white spaces (' ', '\t', '\n', '\f', '\r') omitted;
startsWith(s : String), endsWith(s : String) return a boolean value respectively stating whether self starts/ends with s;
indexOf(s : String), lastIndexOf(s : String) respectively return the index (an integer value) within self of the first/last occurrence of the specified substring s;
split(regex : String) splits the self string around matches of the regular expression regex. Specification of regular expression must follow the definition of Java regular expressions. Result is returned as a sequence of strings;
replaceAll(c1 : String, c2 : String) returns a copy of self in which each occurrence of character c1 is replaced with the character c2. Note that both c1 and c2 are specified as OCL strings. However the function only considers the first character of each of the provided strings;
regexReplaceAll(regex : String, replacement : String) returns a copy of self in which each substring of this string that matches the given regular expression regex is replaced with the given replacement. Specification of regular expression must follow the definition of Java regular expressions.

As a last point, ATL currently defines two additional functions that make it possible to write strings to outputs. These functions are useful for redirecting the result of ATL queries, but they may also be used for debugging purposes:

writeTo(fileName : String) enables to write the self string into the file identified by the string fileName. Note that this string may encode either a full or a relative path to the file. In the last case, the path is relative to the \eclipse directory from which the ATL tool kit is run. If the identified file already exists, the function writes the new content over this existing file;
println() writes the self string onto the default output, that is the Eclipse console.

Note that these two functions are provided as temporary solutions as the ATL toolkit does still not provide any integrated solution for the redirection of the result of ATL queries. They are likely to be removed from future releases of the ATL tool suite.

Numerical data type operations

The following OCL operations are defined for both OCL numerical data types (integer and real):

comparison operators: <, >, =>, =<;
binary operators: *, +, -, /, div(), max(), min();
unary operator: abs().

Note that the - unary operator defined by OCL (that returns the negative value of self) is not implemented in current version of ATL. As a consequence, a -x negative numerical value has to be declared as the result of a call to the - binary operator: 0-x.

OCL also defines some operations that are specific to the integer and the real data types:

integer operation: mod();
real operations: floor(), round().

Besides the OCL-defined operations, ATL provides a set of additional functions. The toString() operation, available for both the integer and real data types returns a string representing the integer/real value of self. There also exist a set of ATL operations specific to the real data type:

cos(), sin(), tan(), acos(), asin();
toDegrees(), toRadians();
exp(), log(), sqrt().

Examples

In the following, some usage examples of OCL operations on primitive data types are illustrated:

testing whether a string is of type OclAny: 'test'.oclIsTypeOf(OclAny)
- evaluates to false
testing whether a string is of kind OclAny: 'test'.oclIsKindOf(OclAny)
- evaluates to true
boolean operations: true or false
- evaluates to true
computing a substring of a given string: 'test'.substring(2, 3)
- evaluates to 'es'
casting a string into upper case: 'test'.toUpper()
- evaluates to 'TEST'
casting a string into a sequence: 'test'.toSequence()
- evaluates to Sequence{'t', 'e', 's', 't'}
checking whether a string ends by a given substring: 'test'.endsWith('ast')
- evaluates to false
getting last index of character "t" in string "test": 'test'.lastIndexOf('t')
- evaluates to 4
replacing character "t" by character "o" in string "test": 'test'.replaceAll('t', 'o')
- evaluates to 'oeso'
replacing occurrences of regular expression "a*" by string "A" in string "aaabaftaap": 'aaabaftaap'.regexReplaceAll('a*', 'A')
- evaluates to 'AbAftAp'
integer division: 23 div 2 or 23."div"(2)
- evaluates to 11
real division: 23/2
- evaluates to 11.5

Collection data types

OCL defines a number of collection data types that provide developers with different ways to handle collections of elements. The provided collection types are Set, OrderedSet, Bag and Sequence. Collection is the common abstract superclass of these different types of collections.

The existing collection classes have the following characteristics:

Set is a collection without duplicates. Set has no order;
OrderedSet is a collection without duplicates. OrderedSet is ordered;
Bag is a collection in which duplicates are allowed. Bag has no order;
Sequence is a collection in which duplicates are allowed. Sequence is ordered.

A collection can be seen as a template data type. This means that the declaration of a collection data type has to include the type of the elements that will be contained by the type instances. Whatever the type of the contained elements, the declaration of a collection data type has to conform to the following scheme:

collection_type(element_datatype)

The supported collection data types are Set, OrderedSet, Sequence and Bag. The element data type can be any supported oclType, including another collection type.

The definition of a collection variable is achieved as follows:

collection_type{elements}

Please note that the brackets used in the type definition must here be replaced by curly brackets. Examples of collection type definitions and instantiations can be found here.

Operations on collections

ATL provides a large number of operations in the context of the different supported collection types. Note that there exists a specific syntax for invoking an operation onto a collection type:

self->operation_name(parameters)

The different kinds of existing OCL collections share a number of common operations:

size() returns the number of elements in the collection self;
includes(o : oclAny) returns a boolean stating whether the object o is part of the collection self;
excludes(o : oclAny) returns a boolean stating whether the object o is not part of the collection self;
count(o : oclAny) returns the number of times the object o occurs in the collection self;
includesAll(c : Collection) returns a boolean stating whether all the objects contained by the collection c are part of the self collection;
excludesAll(c : Collection) returns a boolean stating whether none of the objects contained by the collection c are part of the self collection;
isEmpty() returns a boolean stating whether the collection self is empty;
notEmpty() returns a boolean stating whether the collection self is not empty;
sum() returns a value that corresponds to the addition of all elements in self. These elements must be of a type that support the + operation.

Note that the product() operation defined by OCL is unsupported by the current ATL implementation. However, ATL defines three additional operations in the context of a collection (OCL defines similar operations in the context of each collection type):

asBag() returns a bag containing the elements of the self collection. Order is lost from a sequence or an ordered set. Has no effect in the context of a bag;
asSequence() returns a sequence containing the elements of the self collection. Introduces an order from a bag or a set. Has no effect in the context of a sequence;
asSet() returns a set containing the elements of the self collection. Order is lost from a sequence or an ordered set. Duplicates are removed from a bag or a sequence. Has no effect in the context of a set.

Note that, in the current ATL version, the casting operation asOrderedSet() defined by OCL is implemented for none of the collection types.

Sequence data type operations

The sequence type supports all the collection operations. OCL defines a number of additional operations that are specific to sequences:

union(c : Collection) returns a sequence composed of all elements of self followed by the elements of c;
flatten() returns a sequence directly containing the children of the nested subordinate collections contained by self;
append(o : oclAny) returns a copy of self with the element o added at the end of the sequence;
prepend(o : oclAny) returns a copy of self with the element o added at the beginning of the sequence;
insertAt(n : Integer, o : oclAny), returns a copy of self with the element o added at rank n of the sequence;
subSequence(lower : Integer, upper : Integer) returns a subsequence of self starting from rank lower to rank upper (both bounds being included);
at(n : Integer) returns the element located at rank n in self;
indexOf(o : oclAny) returns the rank of first occurrence of o in self;
first() returns the first element of self (oclUndefined if self is empty);
last() returns the last element of self (oclUndefined if self is empty);
including(o : oclAny) returns a copy of self with the element o added at the end of the sequence;
excluding(o : oclAny) returns a copy of self with all occurrences of element o removed.

Set data type operations

Set supports all collection operations and some specific ones:

union(c : Collection) returns a set composed of the elements of self and the elements of c with duplicates removed (they may appear within c, and between c and self elements);
intersection(c : Collection) returns a set composed of the elements that appear both in self and c;
operator - (s : Set) returns a set composed of the elements of self that are not in s;
including(o : oclAny), returns a copy of self with the element o if not already present in self;
excluding(o : oclAny), returns a copy of self with the element o removed from the set;
symetricDifference(s : Set) returns a set composed of the elements that are in self or s, but not in both.

Note that the flatten() operation defined by OCL is not implemented in the current version of ATL.

OrderedSet data type operations

The sequence type supports all the collection operations. OCL defines a number of additional operations that are specific to ordered sets:

append(o : oclAny) returns a copy of self with the element o added at the end of the ordered set if it does not already appear in self;
prepend(o : oclAny) returns a copy of self with the element o added at the beginning of the ordered set if it does not already appear in self;
insertAt(n : Integer, o : oclAny), returns a copy of self with the element o added at rank n of the ordered set if it does not already appear in self;
subOrderedSet (lower : Integer, upper : Integer) returns a subsequence of self starting from rank lower to rank upper (both bounds being included);
at(n : Integer) returns the element located at rank n in self;
indexOf(o : oclAny) returns the rank of first occurrence of o in self;
first() returns the first element of self (oclUndefined if self is empty);
last() returns the last element of self (oclUndefined if self is empty).

Besides this set of operations specified by OCL, ATL implements the following additional functions:

union(c : Collection) returns an ordered set composed of the elements of self followed by the elements of c with duplicates removed (they may appear within c, and between c and self elements);
flatten() returns an ordered set directly containing the children of the nested subordinate collections contained by self;
including(o : oclAny) returns a copy of self with the element o added at the end of the ordered set if it does not already appear in self;
excluding(o : oclAny) returns a copy of self with the o removed.

Bag data type operations

The bag operations defined by the OCL specification are not available with the current ATL implementation.

Iterating over collections

The OCL specification defines a number of iterative operations, also called iterative expressions, on the collection types. The main difference between a classical operation and an iterative expression on a collection is that the iterator accepts an expression as parameter, whereas operations only deal with data. The definition of an iterative expression includes:

the iterated collection, which is referred as the source collection;
the iterator variables declared in iterative expressions, which are referred as the iterators;
the expression passed as parameter to the operation, which is referred as the iterator body.

The syntax used to call an iterative expression is the following:

source->operation_name(iterators | body)

ATL currently provides support for the following set of defined iterative expressions:

exists(body) returns a boolean value stating whether body evaluates to true for at least one element of the source collection;
forAll(body) returns a boolean value stating whether body evaluates to true for all elements of the source collection;
isUnique(body) returns a boolean value stating whether body evaluates to a different value for each element of the source collection;
any(body) returns one element of the source collection for which body evaluates to true. If body never evaluates to true, the operation returns OclUndefined;
one(body) returns a boolean value stating whether there is exactly one element of the source collection for which body evaluates to true;
collect(body) returns a collection of elements which results in applying body to each element of the source collection;
select(body) returns the subset of the source collection for which body evaluates to true;
reject(body) returns the subset of the source collection for which body evaluates to false (is equivalent to select(not body));
sortedBy(body) returns a collection ordered according to body from the lowest to the highest value. Elements of the source collection must have the < operator defined.

Note that the collect() operation provided by ATL implements the semantics of the collectNested() operation defined in the OCL specification. Getting the semantics of the collect() operation as defined by OCL can simply be achieved with ATL by calling the flatten() operation onto the result provided by the ATL collect() iterative expression, as follows:

source->collect(iterator | body)->flatten()

The ATL language introduces another constraint compared to the OCL specification. The specification indeed allows declaring multiple iterators in the scope of the exists() and the forAll() iterative expressions. This feature is not supported by the current ATL implementation, in which the number of iterator is limited to one, whatever the considered iterative expression.

Besides these predefined iterative operations, OCL specifies a more generic collection iterator, named iterate(). This iterative expression has an iterator, an accumulator and a body. The accumulator corresponds to an initialized variable declaration. The body of an iterate() expression is an expression that should make use of both the declared iterator and accumulator. The value returned by an iterate() expression corresponds to the value of the accumulator variable once the last iteration has been performed. An iterative expression is defined with the following syntax:

source->iterate(iterator, variable_declaration = init_exp |
	body
)

Examples

In the following, some operations on collections are illustrated:

declaring the sequence of integer type: Sequence(Integer)
specifying a sequence of integers: Sequence{1, 2, 3}
declaring the set of sequences of string type: Set(Sequence(String))
specifying a set of sequences of strings: Set{Sequence{'monday'}, Sequence{'march', 'april', 'may'}}
testing whether a bag is empty: Bag{1, 2, 3}->isEmpty()
- evaluates to false
testing whether a set contains an element: Set{1, 2, 3}->includes(1)
- evaluates to true
testing whether a set contains all the elements of another set: Set{1, 2, 3}->includesAll(Set{3, 2})
- evaluates to true
getting the size of a sequence: Sequence{1, 2, 3}->size()
- evaluates to 3
- note that Set{3, 3, 3}->size() evaluates to 1 since the set data type eliminates duplicates
getting the first element of an ordered set sequence: OrderedSet{1, 2, 3}->first()
- evaluates to 1
computing the union of two sequences: Sequence{1, 2, 3}->union(Sequence{7, 3, 5})
- evaluates to Sequence{1, 2, 3, 7, 3, 5}
computing the union of two sets: Set{1, 2, 3}->union(Set{7, 3, 5})
- evaluates to Set{1, 2, 3, 7}
flattening a sequence of sequences: Sequence{Sequence{1, 2}, Sequence{3, 5, 2}, Sequence{1}}->flatten()
- evaluates to Sequence{1, 2, 3, 5, 2, 1}
computing a subsequence of a sequence: Sequence{Sequence{1, 2}, Sequence{3, 5, 2}, Sequence{1}}->subSequence(2, 3)
- evaluates to Sequence{ Sequence{3, 5, 2}, Sequence{1}}
inserting an element at a given position into a sequence: Sequence{5, 15, 20}->insertAt(2, 10)
- evaluates to Sequence{5, 10, 15, 20}
computing the intersection of two sets: Set{1, 2, 3}->intersection(Set{7, 3, 5})
- evaluates to Set{3}
computing the symmetric difference of two sets: Set{1, 2, 3}->symetricDifference(Set{7, 3, 5})
- evaluates to Set{1, 2, 7, 5}
selecting all elements of a sequence that are smaller or equal to 3: Sequence{1, 2, 3, 4, 5, 6}->select(i | i <= 3)
- evaluates to Set{1, 2, 3}
collecting the names of all MOF classes: MOF!Class.allInstances()->collect(e | e.name)
- checking whether all the numbers in a sequence are greater than 2: Sequence{12, 13, 12}->forAll(i | i > 2)
- evalutes to true
checking whether there is only one element of the sequence that is greater that 2: Sequence{12, 13, 12}->one(i | i > 2)
- evalutes to false
checking whether there exists a number in the sequence that is greater than 2: Sequence{12, 13, 12}->exists(i | i > 2)
- evaluates to true
computing the sum of the positive integer of a sequence using the iterate instruction:Sequence{8, -1, 2, 2, -3}->iterate(e; res : Integer = 0 | if e > 0 then res + e else res endif )
- evaluates to 12;
- is equivalent to Sequence{8, -1, 2, 2, -3}->select(e | e > 0)->sum()

Enumeration data types

An enumeration is an OclType. It has a name just as any other data type. However, compared to the data presented up to now, the enumerations have to be defined within the source and target metamodels of a transformation.

With the OCL specification, referring to an enumeration literal (e.g. an enumeration defined value) is achieved by specifying the enumeration type (e.g. the name of the enumeration), followed by two double-points and the enumeration value. Consider, as an example, an enumeration named Gender that defines two possible values, male and female. Accessing to the female value of this enumeration type in OCL is achieved as follows: Gender::female.

The current ATL implementation differs from the OCL specification. Access to enumeration values is simply achieved by prefixing the enumeration by a sharp character (the enumeration type is no more required): #female. The enumeration data type is associated with no specific operation.

Tuple data type

The tuple data type enables to compose several values into a single variable. A tuple consists into a number of named parts that may each have a distinct type. Note that a tuple type is not named. As a consequence, a declared tuple type has to be identified by its full declaration each time it is required.

Each part of a tuple type is associated with an OclType and is identified by a unique name. The declaration of a tuple data type must conform to the following syntax:

TupleType(var_name1 : var_type1, ..., var_nameN : var_typeN)

Note that the order in which the different parts are declared is not significant. As an example, it is possible to consider the declaration of a tuple type associating an Author model element from the MMAuthor metamodel with a couple of strings encoding the title of a book and the name of the editor of this book:

TupleType(a : MMAuthor!Author, title : String, editor : String)

The instantiation of a declared tuple variable has to respect the following syntax:

Tuple{var_name1 [: var_type1]? = init_exp1, ..., var_namen [: var_typen]? = init_expn}

When declaring a tuple instance, the types of the tuple parts can be omitted. As a consequence, the two following tuple instantiations corresponding to the tuple type defined above are equivalent:

Tuple{editor : String = 'ATL Eds.', title : String = 'ATL Manual', a : MMAuthor!Author = anAuthor}
Tuple{title = 'ATL Manual', a = anAuthor, editor = 'ATL Eds.'}

As for the declaration of a tuple type, the instantiation of the different parts of a tuple variable may be performed in any order. The different parts of a tuple structure can be accessed using the same dot notation that is used for the invocation of operations or the access to model element attributes. Thus, the expression

Tuple{title = 'ATL Manual', a = anAuthor, editor = 'ATL Eds.'}.title

provides access to the title part of the tuple.

Besides the set of common operations, the current ATL implementation defines an additional casting operation in the context of the tuple dada type: the asMap() operation returns a map variable in which the name of the tuple parts are associated with their respective values.

Map data type

Provided as an additional facility in the ATL implementation, the map data type does not belong to the OCL specification. This data type enables to manage a structure in which each value is associated with a unique key that enables to access it (see the Java Map interface for further details).

The declaration of a map type has to conform to the following syntax:

Map(key_type, value_type)

Note that, as a tuple type, a map type is not named, which again implies to specify the full type declaration when required. The following map declaration associates some Author model element values with integer keys:

Map(Integer, MMAuthor!Author)

Instantiating a map variable is achieved according to the following syntax:

Map{(key1, value1), ..., (keyn, valuen)}

As an example, the following expression instantiates a two entries map corresponding to the map type declared above:

Map{(0, anAuthor1), (1, anAuthor2)}

Besides the set of common operations, the ATL implementation provides the following operations on map data:

get(key : oclAny) returns the value associated with key within the self map (or OclUndefined if key is not a key of self);
including(key : oclAny, val : oclAny) returns a copy of self in which the couple (key, val) has been inserted if key is not already a key of self;
union(m : Map) returns a map containing all self elements to which are added those elements of m whose key does not appear in self;
getKeys() returns a set containing all the keys of self;
getValues() returns a bag containing all the values of self.

Model element data type

The last kind of data type introduced by the OCL specification corresponds to the model elements. These last are defined within the source and target metamodels of an ATL transformation. Metamodels usually define a number of different model elements (also called classes).

In ATL, model element variables are referred to by means of the notation metamodel!class in which metamodel identifies (through its name) one of the metamodels handled by the transformation, and class points to a given model element (e.g. class) of this metamodel. Note that, as opposed to the OCL notation, which does not specify the metamodel a given class comes from, the ATL notation makes it possible to handle several metamodel at once.

A model element has a number of features that can be either attributes or references. Both are accessed through the dot notation self.feature. Thus, in the context of the MMAuthor metamodel, the expression anAuthor.name enables to access to the attribute name of the instance anAuthor of the Author class.

In ATL, the model elements can only be generated by means of the ATL rules (either matched or called rules). Initializing a newly generated model element consists in initializing its different features. Such assignments are operated by means of the bindings of the rules target pattern elements.

Please note that the operation oclIsUndefined(), defined for the OclAny data type, tests whether the value of an expression is undefined. This operation is useful when applied on an attribute with a multiplicity zero to one (which is void or not). However, attributes with the multiplicity n are usually represented as collections that may be empty and not void.

Full name reference to metamodel classes

It is also possible to include the full path using the following scheme:

<Package1Name>::<Package2Name>::<ClassifierName>

Actually the ATL Parser doesn't deal well with "::" so we need to surround the path using ".

For instance, using the metamodel excerpt given above, we could write:

MM!"P1::C1"
MM!"P1::P2::C2"
MM!"P3::C3"

In some cases, full name reference is required to avoid ambiguity due to name collision. Let us consider the following metamodel:

package P1 {

  class C1 {}

  package P2 {
    class C1 {}
  }
}

package P3 {
  class C1 {}
}

Using MM!C1 is incorrect because it cannot reliably me mapped to a specific class. If you try to do this, a warning will be reported in the ATL console. In this case, it is mandatory to write:

MM!"P1::C1"
MM!"P1::P2::C1"
MM!"P3::C1"

Examples

Here is a sample of OCL expressions using features of model elements. They are defined in the context of the MOF metamodel:

collect the names of all MOF classes:

MOF!Class.allInstances()->collect(e | e.name)

getting the names of all primitive MOF types by filtering:

MOF!DataType.allInstances()
       ->select(e | e.oclIsTypeOf(MOF!PrimitiveType))
       ->collect(e| e.name)

getting the names of all primitive MOF types the simple way:

MOF!PrimitiveType.allInstances()->collect(e| e.name)

an enumeration instance in MOF:

MOF!VisibilityKind.labels

getting the names of all classes inheriting from more than one class:

MOF!Class.allInstances()
       ->select(e | e.supertypes->size() > 1)
       ->collect(e | e.name)

ATL Comments

In ATL, as in the OCL standard, comments start with two consecutive hyphens "--" and end at the end of the line. The ATL editor in Eclipse colours comments with dark green, if the standard configuration is used:

-- this is an example of a comment

OCL Declarative Expressions

Besides the declarative expressions that correspond to the instances of the supported data types, as well as the invocation of operations on these data types, OCL defines additional declarative expressions that aim to enable developers to structure OCL code. This section is dedicated to the description of these declarative expressions. There exist two kinds of advanced declarative expressions: the "if" and the "let" expressions. The "if" expression provides an alternative expression facility. The "let" expression, as for it, enables to define and initialize new OCL variables.

If expression

An OCL "if" expression is expressed with an if-then-else-endif structure. As an expression, an "if" expression should be evaluated (e.g. must have a value) in any cases. This means that the "else" clause of an "if" expression can not be omitted. All "if" expressions must conform to the following syntax:

if condition
then
	exp1
else
       exp2
endif

The condition of the "if" expression is a boolean expression. According to the evaluation of this boolean expression, the "if" expression will return the value corresponding to either exp1 (in case condition is evaluated to true) or exp2 (in case condition is evaluated to false). This is illustrated by the following simple "if" expression:

if 3 > 2
then
	'three is greater than two'
else
	'this case should never occur'
endif

Note that the different parts of an "if" expression can, in turn, include another composed OCL expression, including operation invocations, "let" expressions or nested "if" expressions. As an example, it is possible to consider the following example:

if mySequence->notEmpty()
then
	if mySequence->includes(myElement)
	then
		'the element is at position '
		+ mySequence->indexOf(myElement).toString()
	else
		'the sequence does not contain the element'
	endif
else
	'the sequence is empty'
endif

Let expression

The OCL "let" expression enables the definition of variables. A "let" expression has to conform to the following syntax:

let var_name : var_type = var_init_exp in exp

The identifier var_name corresponds to the name of the declared variable. var_type identifies the type of the declared variable. A variable declared by means of a "let" expression must be initialized with the var_init_exp. The initialization expression can be of any available OCL expression type, including nested "let" expressions. Finally, the in keyword introduces the expression in which the newly declared variable can be used. Again, this expression can be of any existing OCL expression type. This is illustrated by the following simple example:

let a : Integer = 1 in a + 1

Several "let" expressions can be enchained in order to declare several variables, as in the following example:

let x : Real =
	if aNumber > 0
	then
		aNumber.sqrt()
	else
		aNumber.square()
	endif
in let y : Real = 2 in x/y

An OCL variable is visible from is declaration to the end of the OCL expression it belongs to. Note that, although it is not advised, OCL allows developers to declare several variables of the same name within a single expression. In such a case, the lastly declared variable will hide the other variables having the same name.

The "let" expressions also prove to be very useful at the debugging stage. Indeed, the ATL development tools integrate debugging facilities that enable, among other things, to consult the value of the declared variables during the execution of an ATL program. In many cases, it proves to be useful to also be able to consult the value returned by a complex OCL expression. This could be achieved with few modification of the OCL code by declaring an OCL variable initialized with the complex expression to be checked. By this means, the value computed by the expression will be stored in an OCL variable, and thus be available for visualization during the debugging of the ATL program.

In order to illustrate this point, consider the following expression:

aSequence->first().square()

It is here assumed that the collection aSequence is a sequence of Real elements. In case this sequence is empty, the invocation of the operation first() will return the value OclUndefined. Invoked onto OclUndefined, the operation square() will raise an error at runtime. In such a case, it may be interesting to be able to check, at debug stage, whether the first element exists or is undefined by storing its value in a dedicated variable. This is the purpose of the following expression:

let firstElt : Real = aSequence->first() in firstElt.square()

Other expressions

Besides the "if" and "let" structural expressions, the OCL language enables to define different kinds of expressions whose syntax has been introduced in the Data Types section. These expressions include:

the constant expressions, which correspond to a constant value of any supported data type;
the helper/attribute call expressions which correspond to the call of an helper/attribute either defined in the context of the ATL module or of any source model element. The expression is resolved into the value returned by the helper/attribute;
the operation call expressions, which correspond to the call of a standard operation defined for a supported data type. The expression is resolved into the value returned by the operation;
the collection iterative expressions, which correspond to the call of an iterative expression on a supported collection data type. The expression is resolved into the value returned by the called iterative operation.

Expressions tips & tricks

A number of errors, while designing OCL expressions in ATL, come from the evaluation mode of these OCL expressions. Indeed, in many languages, such as C++ and Java, there exists an optimiser that stops the evaluation of logical expressions when finding either a true value followed by the "or" logical operator or a false value followed by the "and" logical operator. No matter the rest of the expression may result into an error or an exception, the expression will be successfully evaluated.

As opposed to these common programming languages, the semantics of composed expressions, as defined by OCL, are such that each expression has to be fully evaluated. As a consequence, some expressions that usually appear to be correct will raise errors in ATL, as illustrated by the following example:

not person.oclIsUndefined() and person.name = 'Isabel'

This expression will therefore raise an error for an undefined person model element when evaluating the expression person.name. An error-free way to express an equivalent logical expression is:

if person.oclIsUndefined()
then
	false
else
	person.name = 'Isabel'
endif

The same remark can be applied similarly to the logical expressions that use the logical "or" operator, such as:

person.oclIsUndefined() or person.name = 'Isabel'

The correct way to express this logical expression is:

if person.oclIsUndefined()
then
	true
else
	person.name = 'Isabel'
endif

Note that the logical expressions that are likely to raise this kind of errors may be embedded in more complex OCL expressions:

collection->select(person | not person.oclIsUndefined() and person.name = 'Isabel')

Using the same rewriting rule, this expression can be transformed into the correct following expression:

collection->select(person |
	if person.oclIsUndefined()
	then
		false
	else
		person.name = 'Isabel'
	endif
)

There may exist several ways to rewrite an incorrect expression. Thus, the following expression will compute the same result:

collection
	->select(person | not person.oclIsUndefined())
	->select(person | person.name = 'Isabel')

Note that the first solution should here be preferred to this one for efficient reasons: the first solution iterates the collection only once.

ATL Helpers

ATL enables developers to define methods within the different kinds of ATL units. In the ATL context, these methods are called helpers. They make it possible to define factorized ATL code that can then be called from different points of an ATL program. There exist two different, although very similar from their syntax, kinds of helpers: the functional and the attribute helpers. Both kinds of helpers must be defined in the context of a given data type. However, compared to an attribute helper, which is commonly referred to as an attribute, a functional helper, referred to as a helper, can accept parameters. This difference implies some differences in the execution semantics of both helper kinds.

Helpers

ATL helpers can be viewed as the ATL equivalent to methods. They make it possible to define factorized ATL code that can be called from different points of an ATL transformation. An ATL helper is defined according to the following scheme:

helper [context context_type]? def : helper_name(parameters) : return_type = exp;

Each helper is characterized by its context (context_type), its name (helper_name), its set of parameters (parameters) and its return type (return_type). The context of a helper is introduced by the keyword context. It defines the kind of elements the helper applies to, that is, the type of the elements from which it will be possible to invoke it. Note that the context may be omitted in a helper definition. In such a case, the helper is associated with the global context of the ATL module. This means that, in the scope of such a helper, the variable self refers to the run module/query itself.

The name of a helper is introduced by the keyword def. As its context, it is part of the signature of the helper (along with the parameters and the return_type). A helper accepts a set of parameters that is specified between brackets after the helper's name. A parameter definition includes both the parameter name and the parameter type, as specified by the following scheme:

parameter_name : parameter_type

Several parameters can be declared by separating them with a comma (","). The name of the parameter (parameter_name) is a variable identifier within the helper. This means that, within a given helper definition, each parameter name must be unique. Note that the specified context type as well as the parameters' type and the return type may be of any of the data types supported by ATL. The body of a helper is specified as an OCL expression. This expression can be of any of the supported expression types. As an example, it is possible to consider the following helper:

helper def : averageLowerThan(s : Sequence(Integer), value : Real) : Boolean =
	let avg : Real = s->sum()/s->size() in avg < value;

This helper, named averageLowerThan, is defined in the context of the ATL module (since no context is explicitly specified). It aims to compute a boolean value stating whether the average of the values contained by an integer sequence (the s parameter) is strictly lower than a given real value (the value parameter). The body of the helper consists in a "let" expression which defines and initializes the avg variable. This variable is then compared to the reference value.

Note that several helpers may have the same name in a single transformation. However, helpers with a same name must have distinct signatures to be distinguishable by the ATL engine (see Limitations).

Calling super helpers

The super keyword lets you call helpers with the same name defined on a super type of the current type.

Suppose you have the following metamodel:

class A {}
class B extends A {}

Then you can write:

helper context A def: test() : Integer = 1;
helper context B def: test() : Integer = super.test() + 1;

Attributes

Besides helpers, the ATL language makes it possible to define attributes. Compared to a helper, an attribute can be viewed as a constant that is specified within a specific context. The major difference between a helper and an attribute definition is that the attribute accepts no parameter.

The syntax used to define an ATL attribute is very close to the definition of functional helpers. The only difference is that the attribute syntax does not enable to define any parameter:

helper [context context_type]? def : attribute_name : return_type = exp;

As for a helper, the context definition can be omitted in the declaration of an attribute. In this case, the attribute will be associated with the ATL module context. The following attribute, which is related to the MMPerson metamodel, can be considered as an example:

helper def : getYoungest : MMPerson!Person =
	let allPersons : Sequence(MMPerson!Person) =
		MMPerson!Person.allInstances()->asSequence() in
	allPersons->iterate(p; y : MMPerson!Person = allPersons->first() |
		if p.age < y.age
		then
			p
		else
			y
		endif
	);

This attribute, named getYoungest, is defined within the ATL module context. It applies to a source metamodel MMPerson that contains Person model elements. It aims to compute the youngest person of the source model (the return type is therefore MMPerson!Person). The attribute body consists in a "let" expression that defines the allPersons variable. This variable is a sequence of MMPerson!Person model elements that contains all the persons defined within the source model (note that the computed set has to be cast into a sequence). The computed sequence is then iterated by means of an iterate expression in which the iteration variable p represents the currently iterated person. The iterate expression results into a MMPerson!Person model element which will correspond to the youngest of the iterated persons. This result is contained by the variable y which is initialized to the first person of the allPersons sequence (in order to get this first person, it is required to define a sequence rather than a set). The body of this iterate expression consists in an "if" expression that simply compares the ages of the current youngest person to the one of the currently iterated person. According to the result of this comparison, the "if" expression will either return the previous youngest person or the iterated one.

Declaring a parameter-less helper and an attribute may appear to be equivalent. However, there exists a major difference between the helpers and the attributes execution semantics. The code of a helper is executed each time this helper is invoked. As opposed to a helper, an attribute accepts no parameter. This means that, for a given execution context (an input model element or the ATL module), an attribute will always return the same value. The ATL engine therefore computes the return value of an attribute only once, either when this attribute is invoked for the first time, or at the transformation/query initialization stage for those attributes that are declared in the context of the ATL module.

Limitations

Current implementation suffers from three limitations in the domain of helpers/attributes. The first one deals with the definition of the signature of the helpers. Helpers are indeed identified through their signature which includes the helper name, its context and its parameters. However, current implementation only considers the subset composed of the helper name and the helper context of this signature: the helpers' parameters do not make it possible to discriminate helpers that have a same name and same context. This implies that all the helpers defined within a given context in an ATL program must have a distinct name. This restriction also concerns the helpers that are defined within a library which is imported in either a query or a module.

The second limitation concerns the definition of helpers in the context of a collection type. Such definitions are actually unsupported by the ATL engine. A simple solution to get round this problem is to move the collection element from context to parameters and to declare the helper in the context of the ATL module. Consider the definition of a helper that aims to select among a set of Person model elements those who are younger than a given age. This helper should be defined as:

helper context Set(MMPerson!Person) def : getYoungPersons(age : Integer) :
	Set(MMPerson!Person) =
	self->select(p | p.age < age);

Taking into account the current ATL limitation, this helper can be defined as follows:

helper def : getYoungPersons(s : Set(MMPerson!Person), age : Integer) :
	Set(MMPerson!Person) =
	s->select(p | p.age < age);

Note that this change has a very limited impact onto the body of the helper. The only difference is the self variable used in the previous version of the helper that has to be replaced by the name of the parameter that represents the collection (s). Finally, last limitation concerning helpers is related to the library unit. Current implementation does not support the definition of attributes within an ATL library. The developer should therefore substitute a parameter-less helper to each of the attributes of the developed libraries. As an example, in the scope of a library, the following attribute:

helper context String def : getFirstChar : String = self.substring(1, 1);

must be replaced by its corresponding helper:

helper context String def : getFirstChar() : String = self.substring(1, 1);

ATL Rules

In the scope of the ATL language, the generation of target model elements is achieved through the specification of transformation rules. ATL defines two different kinds of transformation rules: the matched and the called rules. A matched rule enables to match some of the model elements of a source model, and to generate from them a number of distinct target model elements.

As opposed to matched rules, a called rule has to be invoked from an ATL imperative block in order to be executed. ATL imperative code can be defined within either the action block of matched rules, or the body of the called rules

ATL imperative code

ATL enables developers to specify imperative code within dedicated blocks, either in matched or called rules. An imperative block is composed of sequence of imperative statements. As in the Java C or C++ languages, each statement must be ended with a semicolon character (";").

The current ATL implementation provides three kinds of statements: the assignment statements, the "if" statements and the "for" statements. Note that, as opposed to the OCL expressions, these statements do not return any value. As a consequence, they can not be used in the scope of some ATL declarative code. The three different imperative statements are detailed in the following subsections.

The assignment statement

The ATL assignment statement enables to assign values to either attributes that are defined in the context of the ATL module, or to target model element features. The syntax of the assignment statement conforms to the following scheme:

target <- exp;

As specified, the target of the assignment is either a module attribute or an output model element feature. The assigned expression (exp) can be of any of the supported ATL expressions.

Consider, as a first example, the following attribute definition which defines an integer counter in the context of the ATL module:

helper def: counter : Integer = 0;

The value of this counter attribute can be incremented in the scope of an imperative block using an assignment operation:

thisModule.counter <- thisModule.counter + 1;

The assignment statement can be used in the same way to assign values to model element features in the way. For instance, considering a Person model element aPerson, it is possible to write:

aPerson.father.age <- aPerson.age + 25;

It is possible to initialize the references of a newly generated target model element. The following assignment illustrates this with the assignment of another locally generated (e.g. in the same rule) model element (anotherPerson):

aPerson.father <- anotherPerson;

In the same way, it is also possible to assign to a reference a model element that is generated by a different matched rule. As described here, in such a case, the assigned element is the corresponding source element. If this last does not correspond to a rule default target pattern element, it is required to use the operation resolveTemp(). Note however that the operation resolveTemp shall be called only once the matching phase of the transformation has completed. This means that resolveTemp cannot be invoked neither from the entrypoint called rule, nor from another called rule invoked from this entrypoint.

The if statement

The "if" statement enables to define alternative imperative treatments. "if" statements have to conform to the following syntax:

if(condition) {
	statements1
}
[else {
	statements2
}]?

Each "if" statement defines a condition. This condition must be an OCL expression that returns a boolean value. An "if" statement must also include a "then" statements section. This section, specified between curved brackets, contains the sequence of statements (statements1) that is executed when the conditional expression is evaluated to true. An "if" statement may also include an optional "else" statements section. When specified, this section has to follow the "then" statements section. It is introduced by the keyword else, and must also be defined between curved brackets. This section contains the optional sequence of statements (statements2) that has to be executed when the conditional expression is evaluated to false.

The following example illustrates the use of an "if" statement limited to a simple "then" section:

if(aPerson.gender = #male) {
	thisModule.menNb <- thisModule.menNb + 1;
	thisModule.men->including(aPerson);
}

Next example presents an "if" expression defining both a "then" and an "else" sections, with a nested "if" statement:

if(aPerson.gender = #male) {
	thisModule.fullName <- 'Mr. ' + aPerson.name + ' ' + aPerson.surname;
}
else {
	if(aPerson.isSingle) {
		thisModule.fullName <- 'Miss ' + aPerson.name;
		thisModule.surname <- aPerson.surname;
	}
	else {
		thisModule.fullName <- 'Mrs. ' + aPerson.name;
		thisModule.surname <- aPerson.marriedTo.surname;
	}
	thisModule.fullName <- thisModule.fullName + ' ' + thisModule.surname;
}

Note that the curved brackets delimitating both the "then" and the "else" sections may be omitted when the corresponding sections contain a single statement, as in the following example:

if(aPerson.gender = #male)
	thisModule.men->including(aPerson);
else
	thisModule.women->including(aPerson);

The for statement

The "for" statement enables to define iterative imperative computations. A "for" statement has to conform to the following syntax:

for(iterator in collection) {
	statements
}

The "for" statement defines an iteration variable (iterator) that will iterate over the different elements of the reference collection. For each of these elements, the sequence of statements contained by the "for" statement will be executed.

The following example, also related to the MMPerson metamodel illustrates the use of the "for" imperative statement:

for(p in MMPerson!Person.allInstances()) {
	if(p.gender = #male)
		thisModule.men->including(aPerson);
	else
		thisModule.women->including(aPerson);
}

Current limitations

It is currently not possible to declare variables within ATL imperative blocks. The variables that can be used in the scope of these blocks are:

The source and target model elements declared in the local matched rule;
The target model elements declared in the local called matched rule;
The variables locally declared (e.g. within the rule);
The attributes declared in the context of the ATL module.

Note that the current implantation does not enable to modify the locally defined variables from an imperative assignment statement. This means that, beside the source and target model elements, the only variables that can be modified from an imperative block are the attributes that have been defined in the context of the ATL module. As a consequence, the modifiable variables that may be required in the scope of an imperative bock must, with the current implementation, be declared as ATL module attributes.

Matched Rules

The ATL matched rule mechanism provides ATL developers with a convenient mean to specify the way target model elements must be generated from source model elements. For this purpose, a matched rule enables to specify 1) which source model element must be matched, 2) the number and the type of the generated target model elements, and 3) the way these target model elements must be initialized from the matched source elements. The specification of a matched rule has to conform to the following syntax:

rule rule_name {
	from
		in_var : in_type [(
			condition
		)]?
	[using {
		var1 : var_type1 = init_exp1;
		...
		varn : var_typen = init_expn;
	}]?
	to
		out_var1 : out_type1 (
			bindings1
		),
		out_var2 : distinct out_type2 foreach(e in collection)(
			bindings2
		),
		...
		out_varn : out_typen (
			bindingsn
		)
	[do {
		statements
	}]?
}

Each matched rule is identified by its name (rule_name). A matched rule name must be unique within an ATL transformation. An ATL matched rule is composed of two mandatory (the from and the to parts) and two optional (the using and the do parts) sections. Note that the different variables that may be declared in the scope of a rule (the source and target pattern elements and the local variables) must have a unique name. This restriction does not apply to the OCL expressions contained by this rule. The different sections of an ATL matched rule are detailed in the following subsections.

Source pattern

The from section corresponds to the rule source pattern. This pattern, composed of a single source pattern element contains the source variable declaration (in_var). This declaration specifies the type of the source model elements that will be matched by the rule (in_type). It can moreover contain, between brackets, an optional boolean expression (condition) that enable to target a subset of the transformation source model elements that conform to the source type. If the source pattern element includes no explicit condition, all the source model elements of the transformation that conform to the specified source type will be matched by the rule. The following code excerpt illustrates the syntax of the from section:

from
	p : MMPerson!Person (
		p.name = 'Smith'
	)

Note that the following excerpt

from
	p : MMPerson!Person (
		true
	)

is equivalent to:

from
	p : MMPerson!Person

Local variables section

The optional using section makes it possible to locally declare a number of local variables. The variables declared in this section can be used in the using section itself (provided that the variable is not invoked before its declaration), as well as in the to and the do sections. Each declared variable is identified by its name (vari) and its type (var_typei), and must be initialized using an OCL expression.

The following code excerpt illustrates the use of the using section:

from
	c : GeometricElement!Circle
using {
	pi : Real = 3.14;
	area : Real = pi * c.radius.square();
}

Simple target pattern element

The to section corresponds to the target pattern of the rule. It contains a number of target pattern elements. This section is mandatory and must contain at least one target pattern element. When several target pattern elements are specified, they must be separated by comas (","). Note that the first target pattern element corresponds to the default pattern element of the rule. This means that the target model element associated with this rule's default target pattern can be viewed as the default counterpart of the source model element matched by the rule.

In ATL, there exist two different kinds of target pattern elements: the simple and the iterative ones. Each target pattern element, whatever its type, corresponds to a variable declaration characterized by a name (out_vari) and a type (out_typei). A simple target pattern is specified as a set of bindings that define the way the features (either attributes or references) of the generated element must be initialized. Each binding has to conform to the following syntax:

feature_name <- exp

The name of the initialized feature (feature_name) has to refer to a feature of the variable associated with the target pattern element, as defined in its metamodel. The specified expression (exp) is an OCL expression. When a target pattern element contains more than one binding, the successive bindings have to be separated by comas. Note that it is not required to explicitly initialize all the features of a generated model element. The default value of the features that are not initialized by means of an explicit binding may change according to the model handler used to access the model element. As a consequence, ATL developers are strongly encouraged not to produce code that depends on these default values.

As an example, it is possible to consider the following ATL rule, which is defined in the context of the Biblio metamodel defined on the Biblio metamodel:

rule Journal2Book {
	from
		j : Biblio!Journal
	to
		b : Biblio!Book (
			title <- j.title + '_(' + j.vol + '):' + j.num,
			authors <- j.articles
					->collect(e | e.authors)->flatten()->asSet()
			chapters <- j.articles,
			pagesNb <- j.articles->collect(e | e.pagesNb)->sum()
		)
}

This rule aims to produce a Book model element from a Journal model element. It initializes the title, authors, chapters and pagesNb features of the generated Book:

the title of the Book corresponds to the title of the journal concatenated with its volume (vol) and its number (num);
the chapters of the Book correspond to the model elements that will be generated for the articles of the source Journal;
the authors of the Book correspond to the authors of the different articles of the source Journal, without any duplicate;
the attribute pagesNb is initialized with the sum of the number of pages (pagesNb) of the articles of the source Journal.

This example has illustrated the initialization of the attributes of a generated target model element. As previously stated, the bindings also enable to initialize reference features. Three main cases therefore have to be considered:

assigning to a reference a target model element generated by the current rule;
assigning to a reference the default target model element of another rule;
assigning to a reference a non-default target model element of another rule.

The first case (assigning a model element produced by the same rule) is also the simplest one: the considered reference can be initialized with the name of the other target pattern element. Consider the following example in which the rule Case1 has two target pattern model elements (o_1 and o_2), with o_1 having a reference to a Class2 model element defined (linkToClass2):

rule Case1 {
	from
		i : MM_A!ClassA
	to
		o_1 : MM_B!Class1 (
			linkToClass2 <- o_2
		),
		o_2 : MM_B!Class2 (
			...
		)
}

The reference feature is here simply initialized with the local target pattern element that corresponds to the target model element.

In the second case (assigning the default target element of another rule), the considered reference has to be initialized with the source model element which is matched by the remote rule for generating the target model element to be assigned. In the following example, the rule Case2_R1 aims to generate a target model element (o_1) that has a reference to a target model element that corresponds to the default target pattern (o_1) of the rule Case2_R2. Assuming that the source model element matched by Case2_R1 has a reference (linkToClassB) to the relevant MM_A!ClassB source model element, this assignment is expressed as follows:

rule Case2_R1 {
	from
		i : MM_A!ClassA
	to
		o_1 : MM_B!Class1 (
			linkToClass2 <- i.linkToClassB
		)
}

rule Case2_R2 {
 	from
		i : MM_A!ClassB
	to
		o_1 : MM_B!Class2 (
			...
		),
		...
}

The reference is here initialized with the source model element that is matched by rule Case2_R2 when generating the target model element MM_B!Class2.

It may also happen that a developer wants to initialize a reference with a non-default target pattern element of a remote rule. This last case requires the use of the resolveTemp() operation defined in the context of the ATL module. This operation makes it possible to access the target model elements that are associated with the non-default target pattern elements of a remote rule. It accepts two parameters: the source model element which is matched by the remote rule for generating the target model element to be assigned, and the name of the target pattern element it is associated with. This is illustrated with the following example, which is similar to the previous one, except that the target model element to be assigned is not generated by the default target pattern of rule Case3_R2.

rule Case3_R1 {
	from
		i : MM_A!ClassA
	to
		o_1 : MM_B!Class1 (
			linkToClass2 <- thisModule.resolveTemp(i.linkToClassB, 'o_n')
		)
}

rule Case3_R2 {
	from
		in : MM_A!ClassB
	to
		o_1 : MM_B!Class3 (
			...
		),
		...
		o_n : MM_B!Class2 (
			...
		),
		...
}

Compared to the previous case, the reference is here initialized by invoking the operation resolveTemp() with the source model element (i.linkToClassB, the same that in the previous example) and the name of the target pattern ("o_n") as arguments.

Iterative target pattern element

As opposed to the simple target pattern element, which allows generating a single target model element, the iterative target pattern element makes it possible to generate a set of target model elements conforming to a same type. An iterative target pattern element is introduced by the keyword distinct. It produces a target model element for each element belonging to a given reference ordered collection (either a Sequence or an OrderedSet). This collection, along with its associated iterator (e), is introduced by the keyword foreach. As for a simple target pattern element, an iterative target pattern element defines a number of bindings. These bindings specify the way the features of the target model elements generated by this target pattern element will be initialized.

The following example aims to generate a number of distinct Cell model elements equal to the size of a collection:

using {
	coll : Sequence(String) = Sequence{'a', 'b', 'c'};
}
to
	cells : distinct Table!Cell foreach(e in coll)(
		content <- e,
		id <- coll->indexOf(e)
	)

Note that the collection operation indexOf can be used here to compute a unique column id because the reference collection (coll) does not contain multiple instances of a same element in the collection. Otherwise, the id of the multiple instances of a same element would all have been initialized with the index of the first instance of this element.

Since the reference collection is defined, in this example, as a constant, both its size and its content are known. It is thus possible, instead of using a single iterative target pattern element, to define as many simple target pattern elements as the number of elements in the collection. However, the use of an iterative out pattern element will be required when working with a collection which is a priori unknown (for instance, a collection that comes from a source model).

Attention must be paid when assigning a collection to a target model element feature in the scope of an iterative target pattern element. Indeed, when executing an iterative target pattern element, the ATL engine iterates over the reference collection, but also, in the same time, over the collection expressions that are assigned to features within this pattern element. During the iteration over the reference collection, the current element of a collection expression is assigned to its targeted feature. This has two main consequences:

the assigned collections must have the same size that the reference collection of the target pattern element;
assigning a collection to a feature in the scope of an iterative target pattern element requires to build a collection of collections.

The following example illustrates the way to assign a collection to feature in the scope of an iterative out pattern element:

using {
	coll : Sequence(String) = Sequence{'a', 'b', 'c'};
}
to
	lines : distinct Table!Line foreach(e in coll)(
		id <- coll->indexOf(e),
		cell_titles <-
			Sequence{
				Set{'PlayerA_Score1', 'PlayerB_Score1'},
				Set{'PlayerA_Score2', 'PlayerB_Score2'},
				Set{'PlayerA_Total', 'PlayerB_ Total', 'Total'}
			}
	)

This example is quite similar to the previous one. Instead of generating some Cell model elements, it generates a Line model element for each element of a reference collection (coll). Each line is associated with a unique id, which is computed in the same way it was in the previous example. The difference is here that each line is also characterized by a sequence of strings that encode the title of the different cells of the line.

In order to associate each generated Line model element with its own set of cell titles, the property cell_titles is initialized with a sequence containing as many elements as the reference collection. Each generated line will be associated with its corresponding element in this sequence (the one positioned at the same rank). Thus, the first generated line will be associated with the "PlayerA_Score1" and "PlayerB_Score1" cell titles whereas the third line will be associated with the "PlayerA_Total", "PlayerB_Total" and "Total" cell titles. Please note that:

the type of the assigned collections (here a set) can differ from the type of the collection in which assigned collections are grouped (here a sequence):
- the type of the grouping collection must conform to the type of the reference collection when the defined order has to be respected;
- the type of the assigned collection have to conform to the semantics of the model element being initialized;
the assigned collections are not supposed to have the same size.

Attention must also be paid when referring to the elements generated in the scope of an iterative target pattern. Thus, in the scope of a simple target pattern element, an iterative target pattern variable refers to the whole set of generated elements that are generated by the corresponding pattern element. It is also possible to invoke an iterative target pattern variable from another iterative target pattern element provided that: 1) both iterative target pattern elements belong to the same rule, and 2) both iterative target pattern elements iterate over the same ordered collection. In such a case, the variable refers to the target model element generated by the current iteration.

The following code excerpt illustrates the different ways to refer to elements produced by iterative target pattern elements:

using {
	coll : Sequence(String) = Sequence{'Score1', 'Score2', 'Total'};
}
to
	tab : Table!Table (
		lines <- t_lines
	),
	t_lines : distinct Table!Line foreach(e in coll)(
		id <- coll->indexOf(e),
		caption <- line_captions
	),
	line_captions : distinct Table!Caption foreach(e in coll)(
		content <- e
	)

This new example is inspired from the previous ones. The objective is here to create a Table model element, itself composed of Line model elements. Each Line has to be associated with its own Caption model element. In the scope of the simple target pattern element tab, the variable t_lines refers to the whole sequence of generated Line model elements.

Since both iterative target pattern elements iterate over the same reference collection, the variable line_captions used in the t_lines target pattern element refers to a single of the Caption model elements generated by the line_captions target pattern element. Since the used reference collection is ordered, the line_captions variable will refer to the Caption generated from the same element of the reference collection.

Imperative block section

The last section of an ATL matched rule is the optional do section. This section enables to specify a sequence of ATL imperative statements that will be executed once the initialization of the target model elements generated by the rule has completed. This imperative block can in particular be used to initialize some model element features that have not been initialized using the declarative bindings, or to modify some already initialized features.

The imperative block provides a convenient way to simply assign a unique id to each of the generated model elements. The following example, related to the Biblio metamodel, illustrates this point:

helper def : id : Integer = 0;
...
rule Journal2Book {
	from
		j : Biblio!Journal
	to
		b : Biblio!Book (
			...
		)
	do {
		thisModule.id <- thisModule.id + 1;
		b.id <- thisModule.id;
	}
}

In this example, a global id variable is defined in the context of the ATL module, and initialized to zero. In order to associate each generated model element with a unique id, the imperative block of the matched rule simply increments the value of the id global variable and assigned this new value to the id property of the generated model element.

Lazy Rules

TODO: write lazy rules documentation

How to call lazy rules:

Let a simple lazy rule:

 lazy rule getCross {
   from
     i: ecore!EObject
   to 
     rel: metamodel!Relationship (
     )
 }

We can call it from a matched rule as follows:

 rule Example {
   from 
     s : ecore!EObject
   to 
     t : metamodel!Node (
       name <- s.toString(),
       edges <- thisModule.getCross(s)
 }

If we want to call lazy rule multiple times:

 rule Example {
   from 
     s : ecore!EObject
   to 
     t : metamodel!Node (
       name <- s.toString(),
       edges <- ecore!EClass.allInstancesFrom('yourmodel')->collect(e | thisModule.getCross(e))
  }

Unique Lazy Rules

Declaring a lazy rule as unique, with the following syntax:

unique lazy rule Example{
     ...
}

give it the following behavior:

When a unique lazy rule is executed, it always returns the same target element for a given source element. The target element is retrieved by navigating the internal traceability links, in a way similar to standard rules.

Non-unique lazy rule do not navigate the traceability links, and create new target elements at each execution.

Called Rules

Besides matched rules, ATL defines an additional kind of rules enabling to explicitly generate target model elements from imperative code. Except for the entrypoint called rule, this kind of rules must be explicitly called from an ATL imperative block. The specification of a called rule has to conform to the following syntax:

[entrypoint]? rule rule_name(parameters){
	[using {
		var1 : var_type1 = init_exp1;
		...
		varn : var_typen = init_expn;
	}]?
	[to
		out_var1 : out_type1 (
			bindings1
		),
		out_var2 : distinct out_type2 foreach(e in collection)(
			bindings2
		),
		...
		out_varn : out_typen (
			bindingsn
		)]?
	[do {
		statements
	}]?
}

A called rule is identified by its name (rule_name). A called rule name has to be unique within an ATL transformation, and must not collide with a helper name. Moreover, a called rule cannot be called "main". A called rule can optionally be declared as the transformation entrypoint. An ATL transformation can include one entrypoint called rule. Compared to the other called rules, the entrypoint called rule does not need to be explicitly called: it is implicitly invoked at the beginning of the transformation execution, once the module initialization phase has completed.

A called rule can accept parameters. They have to be specified in the same way they are for helpers. It is composed of three optional sections: the using, the to and the do sections. Compared to a matched rule, a called rule has no from section, and its to section is optional. Note however that the semantics of the available sections are similar to those defined for matched rules:

the using section makes it possible to declare and initialize local variables. A declared variable is visible from the remaining of the using section as well as from the to and the do ones;
the to section corresponds to the target pattern of the called rule. It contains a number of target pattern elements (either simple or iterative target pattern elements). As opposed to a matched rule, there is here no source matched model element whose features may be used in order to initialize the features of the target model elements;
the do section enables to specify an imperative instruction block. If a to section is specified, the imperative block is executed once the computation of the target pattern has completed.

The following code excerpt, provides a called rule example:

helper def: metamodel : KM3!Metamodel = OclUndefined;
...
entrypoint rule Metamodel() {
	to
		t : KM3!Metamodel
	do {
		thisModule.metamodel <- t;
	}
}

This called rule is defined as the transformation entry point. This means that it is executed between the initialization and the matching phases. It generates a Metamodel model element. The code specified within the imperative block makes a variable (metamodel) defined in the context of the ATL module pointing to this model element. By this mean, the generated Metamodel remains accessible for further computation during the transformation.

Rule inheritance

There is two keywords introduced by rules inheritance: abstract and extends. They can be used like this:

 abstract rule A {
   from [fromA]
   using [usingA]
   to [toA]
   do [doA]
 }
 rule B extends A {
   from [fromB]
   using [usingB]
   to [toB]
   do [doB]
 }
 rule C extends B {
   from [fromC]
   using [usingC]
   to [toC]
   do [doC]
 }

When ATL compiles this transformation, it is the same as if you gave this as input:

 rule B {
   from [fromB]
   using [usingB]
   to [toA.bindings union toB.bindings]
   do [doB]
 }
 rule C {
   from [fromC]
   using [usingC]
   to [toA.bindings union toB.bindings union toC.bindings]
   do [doC]
 }

However, there are some limitations and constraints. First, the compiler does not support multiple inheritances and it is not planned to be implemented. Constraints are the following:

sub rules (e.g. B or C) input pattern (i.e. the from part) has to match a subset of its super rule. For instance, if you match particular class in a super rule, you have to have a more restrictive filter or match a sub class.
input pattern variables names have to be the same in super and sub rules.
output pattern variables names have to be the same in super and sub rules for output pattern you want the union.

Here is a complete example to illustrate. It is a KM3-copier, i.e. every model element from the source model is copied as-is to the target model:

 module Copy;
 create OUT : MM from IN : MM;
 
 rule CopyDataType extends CopyClassifier {
   from
     s : MM!DataType
   to
     t : MM!DataType
 }
 
 rule CopyEnumeration extends CopyClassifier {
   from
     s : MM!Enumeration
   to
     t : MM!Enumeration (
       literals <- s.literals
     )
 }
 
 rule CopyParameter extends CopyTypedElement {
   from
     s : MM!Parameter
   to
     t : MM!Parameter
 }
 
 rule CopyReference extends CopyStructuralFeature {
   from
     s : MM!Reference
   to
     t : MM!Reference (
       isContainer <- s.isContainer,
       opposite <- s.opposite
     )
 }
 
 rule CopyTypedElement extends CopyModelElement {
   from
     s : MM!TypedElement
   to
     t : MM!TypedElement (
       lower <- s.lower,
       upper <- s.upper,
       isOrdered <- s.isOrdered,
       isUnique <- s.isUnique,
       type <- s.type
     )
 }
 
 rule CopyOperation extends CopyTypedElement {
   from
     s : MM!Operation
   to
     t : MM!Operation (
       parameters <- s.parameters
     )
 }
 
 rule CopyAttribute extends CopyStructuralFeature {
   from
     s : MM!Attribute
   to
     t : MM!Attribute
 }
 
 rule CopyEnumLiteral extends CopyModelElement {
   from
     s : MM!EnumLiteral
   to
     t : MM!EnumLiteral
 }
 
 rule CopyPackage extends CopyModelElement {
   from
     s : MM!Package
   to
     t : MM!Package (
       contents <- s.contents
     )
 }
 
 rule CopyClass extends CopyClassifier {
   from
     s : MM!Class
   to
     t : MM!Class (
       isAbstract <- s.isAbstract,
       supertypes <- s.supertypes,
       structuralFeatures <- s.structuralFeatures,
       operations <- s.operations
     )
 }
 
 rule CopyClassifier extends CopyModelElement {
   from
     s : MM!Classifier
   to
     t : MM!Classifier
 }
 
 abstract rule CopyModelElement extends CopyLocatedElement {
   from
     s : MM!ModelElement
   to
     t : MM!ModelElement (
       name <- s.name
     )
 }
 
 rule CopyMetamodel extends CopyLocatedElement {
   from
     s : MM!Metamodel
   to
     t : MM!Metamodel (
       contents <- s.contents
     )
 }
 
 abstract rule CopyLocatedElement {
   from
     s : MM!LocatedElement
   to
     t : MM!LocatedElement (
       location <- s.location
     )
 }
 
 rule CopyStructuralFeature extends CopyTypedElement {
   from
     s : MM!StructuralFeature
   to
     t : MM!StructuralFeature (
       subsetOf <- s.subsetOf,
       derivedFrom <- s.derivedFrom
     )
 }

Rules usage

Here are three types of declarative rules:

Matched rules are applied once for each match. A given set of elements may only be matched by one standard rule,
Lazy rules are applied as many times for each match as it is referred to from other rules (possibly never for some matches).
Unique lazy rules are applied at most once for each match, and only if it is referred to from other rules.

The following table summarizes their differences with respect to the number of time they are applied.

Kind of rule	Number of references to source pattern	Number of times the target pattern gets created	Kind of traceability link created
standard	0	1	default or not (using keyword nodefault)
	1	1
	n > 1	1
lazy	0	0	Not default
	1	1
	n > 1	n
unique lazy	0	0	Not default
	1	1
	n > 1	1

In addition, imperative rules may also be used.

Here are a few guidelines about which rules and constructs to use. They may be summarized as: "Make your transformation as complex as necessary but as simple as possible".

Prefer declarative over imperative: only use imperative constructs for the part of a transformation that needs it if it even does.
Prefer simpler constructs over more complex ones:
- Use standard rules when possible, otherwise use unique lazy rules, and use lazy rules only if necessary.
- Only use resolveTemp if necessary.
- Prefer iterators (e.g., select, collect) over iterate.

ATL Queries

Besides module units, ATL enables developers to define queries on model. A query unit accepts a number of source models and produces a single return value of any supported primitive data type. A query unit is composed a single query element along with a number of helpers and attributes that may be defined in the context of either the ATL module or any model element defined within the query source models. Note that an ATL query unit must start with the declaration of its query element. The specification of this query element has to conform to the following syntax:

query query_name = exp;

There is no constraint on the naming of the query element. However, it is advised to give the query element the same name that the file in which it is defined.

The body of the query element (exp) is an OCL expression of any of the supported primitive data types: string, boolean, integer or real. Helpers and attributes defined in the query file (as well as those that belong to imported ATL libraries) can be called in the scope of the body of the query element.

When using the ATL Integrated Development Environment (IDE), developers may be interesting in writing the result of an executed query into a file. This could be easily achieved by producing a string value (other primitive data types will have to be cast into strings) on which the operation writeTo() can be called. As an example, it is possible to consider the following query:

query PersonNb =
	MMPerson!Person.allInstances()->size().toString().writeTo('result.txt');

This query is executed on a MMPerson model containing a number of Person entities. The query first gets the set of all existing Person classes in the model and gets the size of the computed set. In order to write this value in a file, the computed integer value is cast into a string (operation toString()) before being written into the file "result.txt". Note that, although the result is written into a file, the query still returns the computed string.

ATL Keywords

This section provides the list of the ATL reserved keywords. These keywords cannot be used to name variables in any context of an ATL unit (either a module, a query or a library). It is possible to distinguish three kinds of keywords: the constant keywords, the language keywords and the type keywords:

Constant keywords: true, false;
Type keywords: Bag, Set, OrderedSet, Sequence, Tuple, Integer, Real, Boolean, String, TupleType;
Language keywords: not, and, or, xor, implies, module, create, from, uses, helper, def, context, rule, using, derived, to, mapsTo, distinct, foreach, in, do, if, then, else, endif, let, library, query, for, div, refining, entrypoint.

Note that the use of the string "main", which does not belong to the set of language keywords, is restricted. "main" cannot be used to identify (e.g. to name) neither a called rule, nor a helper or an attribute that is defined in the context of the ATL module.

ATL Tips & Tricks

This section aims to highlight some common problems and errors that may be experienced while starting programming with ATL.

In ATL, an element of the input model should not be matched more than once. At present time, this constraint is not verified at compile time, and this kind of errors can lead to unexpected results. A typical case of multiple matching of an input model element appears with the definition, in the input metamodel, of an inheritance link in which the parent entity is not abstract. Here is a simple example of this kind of situation:

The multiple matching problem appears here when trying to respectively match A and B elements by means of two distinct rules (ruleA and ruleB). With an intuitive source pattern such as a : MM!A, ruleA will match purely A elements as well as B elements. Since these last ones are also matched by ruleB, this raises a multiple matching problem.

To solve the problem, the developer has to ensure that ruleA only matches purely A elements. This is achieved by filtering, in the source pattern of ruleA, the type of the elements to be matched by the rule:

rule ruleA {
	from
		a : MM!A (
			a.oclIsTypeOf(MM!A)
		)
	...

The OCL function oclIsTypeOf here tests whether the input model element is an instance of the metamodel element passed as parameter.

The ATL Tools

This section provide a complete description of the ATL Tools, then aims to explain their usage.

Perspectives

In Eclipse, the notion of perspective refers to a workbench configuration that is arranged in order to optimise the handling of a certain task. A workbench is usually composed of several subwindows (called views) and toolkits. ATL is associated with of two specific perspectives: the main ATL perspective and the ATL Debug perspective, which are respectively dedicated to the design and the debugging of ATL transformations. Switching to the ATL, as well as to the other perspectives available on the Eclipse platform, can be achieved by either the perspective buttons available in the thumb index on the top right hand side of your workbench, or by selecting a perspective within the perspectives menu (Menu bar->Window->Open perspective->Other...).

ATL perspective

The ATL perspective is the main perspective for ATL development. It provides all the required features for the creation of ATL projects, ATL transformation files and ATL launch configurations. The perspective also includes a textual editor dedicated to ATL files. The ATL perspective is composed of seven different views: the Navigator, the Editors, the Outline, the Console, the Error Log, the Properties and the Problems views. Here is a screenshot of an ATL project under the ATL perspective.

In its default configuration, the ATL perspective displays the Navigator view on the left side of the window. The Editors view is situated in the top middle part of the windows, whereas the Outline view is positioned on the top left part of the perspective. Finally, the four remaining views (Problems, Properties, Error Log and Console) share the bottom part of the perspective. Note that it is possible to display a given view in the whole perspective by simply double-clicking onto the view title. Moving back to the original perspective configuration is achieved by double-clicking again onto the view title. The different views of the ATL perspective are detailed in the following subsections.

Navigator

Besides browsing the content of the workbench, the Navigator view provides a number of contextual actions on the different contained element it contains. The list of contextual actions, which depends on the type of the selected element, is displayed in a contextual menu obtained by right-clicking on a given element.

Interesting contextual actions available in the Navigator view include:

Creating a new ATL project at the Navigator root (New->ATL Project);
Creating a new ATL file from an ATL project (New->ATL File);
Creating a directory from an ATL project (New->Other...->Simple->Folder);
Running/debugging an ATL file (Run As->Run.../Debug As->Debug);
Open an ATL file with the ATL Editor (Open With->ATL Editor). Since ATL Editor is the default editor for ATL file, it is launched by a simple double-click on the ATL file;
Open an Ecore file with the Sample Ecore Model Editor (Open With->Sample Ecore Model Editor). The Sample Ecore Model Editor is the default editor for Ecore files. As a consequence, it can be launched by double-clicking on an Ecore file;

Note that the content of the files opened from the Navigator view is displayed within the Editors view by means of the selected editor.

Editors

Eclipse facilitates the development of powerful source editors. Thus, besides the default editors provided by Eclipse and by the EMF framework, an ATL editor has been implemented in order to ease the typing of ATL transformations. This editor is the default editor for .atl files. It performs syntax highlighting, displays the position of defined breakpoints, but also performs runtime parsing, compilation and error detection. The problems that are detected at compile-time are underlined by the ATL Editor. Details about these problems are displayed in the Problems view. These details include the type of detected problem (Error, Warning or Style), a textual description of the problem and the positioning of this problem (line and column numbers) in the compiled file. Note that saving modifications of an ATL file that contains a syntactically correct ATL program triggers the compilation of this file, and thus the generation of a new ASM assembler file. An assembler file has the extension .asm and contains the compiled code of the corresponding ATL file.

Note that, when editing an ATL file by means of the ATL Editor, an outline of the ATL transformation is simultaneously displayed within the Outline view.

Outline

The Outline view aims to provide ATL developers with an overview of the structural elements of the file being edited in the Editors view. To this end, the Outline view has to be synchronized with the active tab of the Editors view.

In the scope of an ATL file, the Outline view displays the structure of the currently edited transformation. Adding, from the ATL Editor view, the code for a new structural element such as a rule or a helper operation will automatically lead to a corresponding addition in the Outline view (at latest when the file is saved). Furthermore, cursors of the ATL Editor and the Outline view always point to the same structural element, as illustrated in the following picture. As a consequence, if the cursor is moved in one of them (either the ATL Editor or the Outline), the other view will replace its own cursor correspondingly.

Details about the transformation element selected in the Outline view are displayed in the Properties view.

In the scope of an ATL transformation, the Outline view also enables to position new breakpoints in the transformation code. The definition of a new breakpoint is achieved, from a selected element of the Outline view, by selecting the Add breakpoint option of the contextual menu. The breakpoints defined within the Outline view will be listed in the Breakpoints view available in the ATL Debug perspective. They are marked in the ATL Editor by means of green points.

Problems

The Problems view aims to display the problems (typically some syntax errors) that have been detected within the currently edited file. In the scope of the current ATL tools current implementation, this view is mainly useful for the edition of ATL files. It therefore displays the list of problems that have been detected in an ATL program at compile-time (when the edited file is saved).

The Problems view currently displays two main kinds of Problems in the scope of an ATL transformation:

Error problems, which are raised for invalid ATL statements (for instance, declaring two models with the same name);
Warning problems, which are raised for valid ATL statements that may be source of errors (for instance, declaring a variable that hides an already existing variable).

For each detected problem, the Problems view displays its type (Error or Warning), a short explanation message and the localisation (in terms of line and column number) of the Problem. Note that the corresponding problems are also directly localised in the Editors view.

Error Log

The Error Log view aims to display and log the Eclipse general errors. It is of no particular use for ATL developers, as ATL errors are displayed in the Console view.

Console

The Console view displays the messages that may be written from the ATL code, using for instance the string operation println(). It also displays the error messages that may be raised by the execution of incorrect ATL programs. Note that these displayed error messages may provide useful information while trying to identify errors within faulty ATL transformations.

ATL Debug perspective

The ATL Debug perspective is dedicated to the debugging of ATL transformations. It provides ATL developers with the usual set of debugging facilities:

positioning of breakpoints;
step-by-step transformation execution;
running transformation to the next breakpoint;
display of variables values;
etc...

This section focuses on the organisation of the ATL Debug perspective and the role of the different views that are part of this perspective. For a detailed description of the debugging facilities offered by the perspective, refer to the Debugging ATL section. The ATL Debug perspective is structured into seven distinct views: the Debug, the Variables, the Breakpoints, the Editors, the Outline, the Console and the Tasks views. Here is a screenshot of the ATL Debug perspective.

In its default configuration, the ATL Debug perspective displays the Debug view on the top left side of the window. The Variables and the Breakpoints views share the top right side of the window. The Editors view is displayed on the middle left side, whereas the Outline view is positioned on the middle right side. Finally, the Console and the Tasks view share the bottom part of the perspective.

Debug

The Debug view provides information on the state of operation stack of the transformation currently being debugged. For this purpose, it displays, as root elements, the list of ATL program currently running in debug mode. For each of these programs, it displays the list of running threads. Note here that an ATL transformation is executed within a single thread. In the scope of this thread, the Debug view displays the stack of called operations.

In the previous screenshot, the Debug view provides information on a single ATL execution of the Author2Person transformation. The call stack of the executed thread contains three operations. The operation currently being executed is _applyAuthor(). This operation has been called by the internal _exec()_ operation which has been itself called by the operation main().

The Debug view also provides useful shortcuts for the common debugging operations (Resume, Terminate, Step Into, Step Over, Step return, etc.). These shortcuts are provides as buttons on the right of the view title bar. Their use is further described in the section dedicated to the debugging of ATL programs.

Variables

As previously illustrated, the Variables view is divided into two distinct parts. The top part of the view displays the values of the variables that are visible from the operation currently being selected in the Debug view. This view offers the possibility to browse the reference properties of these visible variables. By this mean, it is possible to access to the value of model elements that are not directly visible in the scope of the current operation, but that are pointed by some of the currently visible model elements.

The bottom part of the Variables view makes it possible for ATL developers to specify and execute requests onto the set of visible variables.

Breakpoints

The Breakpoints view displays the list of the breakpoints that are currently defined in the transformation being executed. This view makes it possible to select, among defined breakpoints, a subset of active breakpoints. It also provides a number of shortcuts dedicated to the management of breakpoints. These shortcuts are provided as buttons on the right of the title bar of the Breakpoints view.

Programming ATL

This section aims to present the different steps of the design and the programming of an ATL transformation with the provided ATL IDE. Executing an ATL transformation obviously requires an ATL transformation file, but also the source and target metamodels as well as the source models of this transformation.

The first step in the process of designing an ATL transformation is to create an ATL project. Source and target metamodels can be imported from different sources. The main task therefore consists in designing the ATL transformation in itself.

Creating an ATL project

The first step in the design of a new ATL transformation is to be positioned under an ATL project. If no ATL project already exists, this first step requires creating a new empty ATL project (New->ATL Project).

This operation triggers the apparition of the ATL Project Creator window in which the name of the project to be created has to be entered. At this stage, it is advised to give the project a sensible name, for instance by concatenating the source metamodel name, the character "2" and the target metamodel name (such as Author2Person). The ATL project creation is then validated by pushing the Finish button.

For each created project, Eclipse creates a project folder in the Navigator view. A newly generated project can be opened by double-clicking onto the project item in the view. It initially contains a .project file. This file contains the Eclipse metadata that are relative to the project.

Content assist

Completion purposes basic templates for rule, helper, from, to, do, using sections. You can also access EMF metamodels informations (from their nsURI, or a relative path).

Completion on helpers

primitive types
some model elements: context, parameter types, output type

Completion on rules

input, output model elements
left-part of bindings

Usage

To make model elements completion available, you have to put some information on the top of the file :

-- @nsURI : the nsURI for a given metamodel, if you want to load a metamodel from the EMF registry,
-- @path : the path of a given metamodel, if you want to dynamically load a metamodel from an ecore file.

Only EMF metamodels are supported. You must specify the relative path of the file into the workspace.

Here is the top of an UML2AnyMM transformation :

-- @path AnyMM=/AnyProject/AnyFolder/AnyMM.ecore
-- @nsURI UML=http://www.eclipse.org/uml2/2.1.0/UML

module Class2Relational;
create OUT : AnyMM from IN : UML;

-- ...transformation helpers and rules

Completion is triggered with the Ctrl + space keys, or when typing a space in a context where some content assist is available.

Creating an ATL file

The ATL IDE provides a specific wizard dedicated to the creation of ATL files. Beginner ATL developers are encouraged to use this wizard to create new ATL files. Experimented developers may find the wizard tool too complex for the creation of very simple transformations. In this case, they may prefer to create their ATL files from scratch. Both procedures are described in the following subsections.

The ATL File Wizard

The ATL File Wizard is launch, from the Navigator view, by selecting the New->ATL File entry in the contextual menu. Note that is command is also available from the File menu of the Eclipse menu bar. This command triggers the apparition the ATL File Wizard window:

TODO: update with the ATL 3.0 file wizard

The ATL File Wizard makes it possible to specify the name of the file to be created, the type of the ATL unit that will be contained by the file (an ATL module, query or library), the name of the source and target model and metamodel variables as well as the name of the libraries that will be required for the ATL program to run. From these data, the wizard generates the ATL file with the header section that corresponds to the provided information.

The ATL wizard window is organized into four sections: HEAD, IN, OUT and LIB. The HEAD section aims to specify the name of the ATL file, the project it is attached to and its type. The project the ATL transformation is attached to can be selected among the list of existing projects in the Navigator view. The name of the file is not restricted. However, it is strongly advised to give ATL files relevant names. A good naming convention is to name ATL files with the name of the source metamodel, followed by the character "2", followed by the name of the target metamodel. The ATL file will be created with the .atl extension in the root folder of the selected project. As a last point, the developer has to select the type of ATL file to be generated among module (for a classical transformation), query and library. Note that, depending on the selected type of ATL unit, the remaining parts of the ATL File Wizard can be totally or partially disabled.

The IN and OUT sections of the wizard window respectively enable to specify the name of the variables associated with the source and target models and metamodels. In each section, the name of the model and the name of the metamodel this model conforms to have to be respectively entered in the Model and the Metamodel fields. A couple defined by this mean is validated with the Add button. The wizard makes it possible to define multiple source and target models/metamodels. Developers must take care, when specifying the name of the model/metamodel variables to give each of them a unique name.

Finally, the LIB section of the window makes it possible to specify the name of the libraries that will be required for executing the ATL program. A distinct use instruction will be included into the generated ATL file for each specified library.

Creating an ATL file from scratch

It is possible, for ATL developers, to edit their ATL files from scratch by themselves. To this end, the first step is to create an empty generic file. The naming of the file to be created should follow the conventions proposed in the previous section. Moreover, the file must here be explicitly associated with the .atl extension.

Once the ATL file has been created, the developer has to manually edit the header section of the ATL file. Note that the constraints on the naming of the declared model and metamodel variables still have to be respected when editing an ATL header from scratch.

Compiling an ATL file

The compilation of an ATL file corresponds to the update of its associated ASM file. This compilation can only be performed if the considered ATL program is syntactically correct. In the scope of the ATL IDE, the compilation policy is based on the default Eclipse compilation policy: compilation is automatically performed in the background when an edited ATL file is saved into an ATL Project (or any project configured with the ATL nature).

Setting up an ATL run launch configuration

Executing an ATL transformation first requires setting up a transformation launch configuration. An ATL launch configuration aims to resume all the information that is required to execute an ATL transformation. This information includes the paths of the file involved in the transformation (e.g. the ATL file, but also the model, metamodel and library files).

The ATL Configuration tab

TODO: update with the ATL 3.0 configuration tab

The Advanced tab

TODO: update with the ATL 3.0 advanced configuration tab

The Common tab

The Common tab offers the ATL developer to configure the execution environment of the designed transformation. The Common tab is divided in four blocks: Save as, Display in favorites menu, Console Encoding, and Standard Input and Output. Here is a screenshot of the Common tab of the run ATL launch configuration wizard.

The Save as section enables to specify whether the launch configuration data have to be saved as a local or a shared file. As a local file, the launch configuration will only be available through the launch configuration window. The launch configuration can also be saved into a file in order to be shared. When selecting this option, the developer has to specify the path to the launch configuration file (the file has to be saved within the current project). When saved as a shared file, the launched configuration file appears at the specified location. This file, which is an XML file, has the name of the considered transformation with the .launch file extension. Thus, saving the launch configuration in the scope of the current example will trigger the creation of the file Author2Person.launch.

The Display in favorites menu section enables ATL developers to customize the perspective by choosing whether they want a shortcut to the designed launch configuration to appear in the Run and/or Debug menus.

The Console Encoding section enables to select the encoding type of the Console that will be used by the transformation for standard inputs and outputs.

Next section deals with these standard inputs and outputs. It provides developers with the possibility to select the input and output facilities for the ATL program. In this scope, it is possible to allocate a console (default option) and/or a file. The developer can also choose to allocate both a console and a file or, at the opposite, to provide no standard input/output facilities to the transformation. Note that, when specifying a file as standard output, the developer can choose to append the results of the successive transformation executions to the output file.

The last option defined in the Common tab enables to select whether the ATL program has to be executed in background (default option) or not.

Module superimposition

Description

While ATL transformation modules and queries are normally run by themselves, that is one transformation module or query at a time, it is also possible to superimpose several transformation modules on top of eachother. The end result is a transformation module that contains the union of all transformation rules and all helpers, where it is possible for a transformation module to override rules and helpers from the transformation modules underneath. Below is an example of a typical use case for superimposition: the transformation rules of the UML2Copy [1] module are reused and overridden where necessary by the UML2Profiles [2] module.

The UML2Copy transformation module includes a transformation rule for every meta-class instance it must copy. This amounts to approximately 200 rules for the entire UML2 meta-model [3].

Any refinement transformation basically needs to copy all meta-class instances, except for the few meta-class instances that are refined. The UML2Profiles transformation module applies a profile to the "uml::Model" instance, provided it was not yet applied. All other elements should just be copied.

To achieve this, the UML2Profiles module is superimposed on the UML2Copy module. It overrides the "Model" rule, which copies each "uml::Model" instance, by a version that checks that the profile we want to apply has already been applied. It also introduces a new rule "ModelProfile", which checks that the profile we want to apply has not been applied and then applies the profile. The resulting transformation contains all rules from the above figure that have not been striked through.

Note that superimposition is a load-time construct: there is no real transformation module that represents the result of superimposing several modules on top of eachother. Instead, several modules are simply loaded on top of eachother, overriding existing rules and adding new rules.

Usage

TODO: update with ATL 3.0 ant tasks

ATL Superimposition is configured in the Eclipse "Run..." dialog, in the Advanced tab to be exact. You can also use Superimposition from AM3 Ant scripts. An example Ant script can be found here (see the "profiles" macro).

See also the ATL FAQ entry on Superimposition.

TODO: update with ATL 3.0 launch configuration

Note: when adding superimposed modules in the Advanced tab, they override the "main" module specified in the ATL Main Configuration Tab.

Running an ATL launch configuration

Once the launch configuration of a transformation has been correctly fulfilled, it can be run as many times as needed without requiring any change to the configuration. In order to the run a designed ATL transformation, the developer just has to go back to the configuration Run window, to select the created transformation in the ATL Transformation folder (on the left column) and click on the Run button. The other option for running an existing ATL launch configuration is to define shortcuts for this configuration. This could be achieved from the Common tab of the ATL run launch configuration by selecting the Run option within the Display in favourites menu section.

ATL ant tasks

TODO: write ATL ant tasks documentation

Sample build.xml, launches public2private example:

To launch this file, you must use a specific ant configuration: right-click->Run as->Ant Build... then set:

JRE->Run in same JRE as the workspace
Refresh->the entire workspace

<?xml version="1.0"?>
<project name="Public2Private" default="run" basedir=".">
	<property name="samplePath" value="../model/sample.uml" />

	<target name="run">
		<atl.loadModel name="UML" metamodel="MOF" nsURI="http://www.eclipse.org/uml2/2.1.0/UML" />
		<atl.loadModel name="sample" metamodel="UML" path="${samplePath}" />

		<atl.launch path="../transformation/public2private.asm" refining="true">
			<inoutmodel model="sample" name="IN" />
		</atl.launch>

		<atl.saveModel model="sample" path="${samplePath}" />
	</target>

</project>

Debugging ATL

This section aims to introduce the debugging facilities provided by the ATL IDE. The ATL development environment therefore offers ATL developers a dedicated ATL Debug perspective. This perspective provides developers with the most common debugging facilities, including step-by-step transformation execution, running a transformation to the next breakpoint, display of the variables content, etc. Moreover, the ATL IDE enables developers to know, at any time, the ATL instructions currently being executed by highlighting the corresponding code in the ATL Editor.

The ATL debugging operations are available from the ATL Debug perspective. As for a Java program, debugging an ATL transformation implies to execute this transformation in debug mode. This supposes developers to create an ATL debug launch configuration for the transformation. The debug execution mode, along with its associated debugging actions, is triggered by the execution of this debug launch configuration.

Managing breakpoints

The ATL debugging mode makes it possible to define breakpoints within any kind of ATL units, including the libraries that are imported from other ATL units. These breakpoints have to be positioned by means of the Outline view, which is available from both the ATL and the ATL Debug perspectives. Note that, the Outline view only displays the structure of ATL units that are edited with the ATL Editor.

Setting/Removing breakpoints

In the scope of the ATL IDE, the setting of breakpoints in ATL programs can only be achieved through the Outline view. Remember that the Outline view displays the structure of the ATL file currently being edited with the ATL Editor (as a matter of fact, it displays the ATL model corresponding to the edited ATL file). A new breakpoint can be defined at the level of an ATL structural element by selecting the Add breakpoint entry in the contextual menu of the selected element. This is illustrated in the next screenshot in which a breakpoint is positioned at the level of a NavigationOrAttributeCallExp element. Note that the code corresponding to the element selected in the Outline view is simultaneously highlighted in the ATL Editor view.

The Outline view currently allows developers to associate breakpoints with any kind of the structural element of an ATL program. However, positioning a new breakpoint only makes sense for those structural elements that are associated with executed instructions. Structural elements that constitute relevant targets for breakpoints roughly correspond to the OCL expressions that are evaluated by the ATL engine. This means that transformation elements such as a MatchedRule (or a CalledRule) element, a Helper element, or InPattern and OutPattern elements should not be associated with breakpoints. Note that the Outline view allows defining breakpoints for these elements, but they will be ignored during the debugging of the program.

Defined breakpoints appear in the left column of the ATL Editor view. This is illustrated by the following screenshot in which the breakpoint previously positioned onto a NavigationOrAttributeCallExp element is localized by a blue circle in the left column of the ATL Editor. Although the ATL Editor displays the position of the defined breakpoints, it does not enable to handle them. This must be achieved by means of the Breakpoints view of the ATL Debug perspective.

Defined breakpoints can only be removed from the Breakpoints view of the ATL Debug perspective. This view makes it possible to select a number of breakpoints among defined ones. These breakpoints can be removed using the Remove Select Breakpoints button.

Note that breakpoints removal actions are also available in the contextual menu when selecting breakpoints from the breakpoints list (in the Breakpoints view).

Activating/Deactivating breakpoints

The Breakpoints view also offers the possibility to activate and deactivate defined breakpoints. Deactivated breakpoints will not be considered while debugging an ATL transformation. This facility makes it possible to ignore some of the defined breakpoints without having to remove them.

Breakpoint activation/ deactivation is only available from the contextual menu associated with the elements of the breakpoints list. Note that, as breakpoints setting and removal, activation/deactivation can either be performed before or during the debugging of an ATL program.

Limitations

Beside the fact that the Outline view allows defining breakpoints on irrelevant locations, the ATL development environment currently offers poor support in updating the position of already defined breakpoints when an ATL file is compiled (the default ATL compiling policy is to compile files at save-time). It may therefore appear, once an ATL file for which breakpoints are defined has been compiled, that the defined breakpoints point to irrelevant locations in the considered program file. This could materialize by internal errors while debugging the ATL unit.

Creating an ATL Debug launch configuration

As for the run mode, executing an ATL transformation in debug mode first requires to set up an ATL Debug launch configuration. Creating of a new ATL debug launch configuration is achieved, from the Navigator view, by selecting an ATL file in the Navigator view and selecting the 'Debug As->Debug... entry of its contextual menu. Note that this debug launch configuration wizard can also be launched from the Eclipse menu bar by selecting the Debug... entry of the Debug menu.

ATL programs share a common launch configuration for both the run and debug modes. This has two consequences. First, this means that once the run launch configuration of an ATL unit has been configured, there is no need for creating a new launch configuration dedicated to the debug mode. The second consequence is that both kinds of launch configuration must be configured in the same way (except for the disassembly mode option, see below).

The Disassembly mode option available in the ATL Configuration tab of the launch configuration has no effect in run mode. However, in debug mode, this option makes it possible for developers to debug an ATL unit from its bytecode (e.g. contained by the ASM file associated with the ATL program). This debug mode is mainly provided for developers of the ATL language and is out of the scope of this manual.

Running an ATL Debug launch configuration

Executing an ATL debug launch configuration follows the same scheme that for an ATL run launch configuration: from the configuration Debug window, the developer just has to select a transformation in the ATL Transformation folder (on the left column) and click on the Debug button.

As for the run mode, there exists another option which consists in defining a debug shortcut for this configuration. This could be achieved from the Common tab of the ATL launch configuration by selecting the Debug option within the Display in favourites menu section.

Debugging actions

While debugging a program, developers are used to be offered a set of standard debugging actions. In the scope of the ATL IDE, the Debug view of the ATL Debug perspective provides shortcuts to the main debugging operations. While debugging a transformation, the debugging actions can also be reach from the Run menu of Eclipse menu bar and from the contextual menu of either the current thread or its content:

The Resume action triggers the execution of the debugged transformation up to the following breakpoint. A program containing no breakpoint will be executed up to termination.

The Step Over action is a step-by-step action. Activating this action triggers the execution of the current instruction. Note that if this instruction is an operation call (an element of type OperationCallExp in the Outline view), the debugger will step over the execution of the call operation. In the same way, if the current instruction is the last one of the currently executed operation, the debugger will resume to the calling operation.

The Step Into action is another step-by-step action. Triggered onto an expression call instruction, it jumps into the body (e.g. the first instruction) of the called operation. Note that when called onto an instruction that is not an operation call, this Step Into action will behave in the same way that the Step Over one.

The last step-by-step action is the Step Return action. This action resumes the transformation execution up the point from which the current operation was called. Triggered from either a helper, an attribute or a called rule, the Step Return action will resume to the calling user code. Triggered from a source pattern element, the action will resume to the generated main operation __exec__() that will, in turn, call either the next __match operation or the first __exec operation. Finally, triggered from a target pattern element, the action will resume to the generated main operation __exec__() that will, in turn, either call the next __exec operation or run up to the program termination. Note that called from the last instruction of a celled operation, this action behaves in the same way that the previous ones.

The Terminate and Remove action terminates the transformation being debugged, and removes it from the Debug view.

The Remove All Terminated Launches action removes all terminated transformation from the Debug view. This action is not available if the view contains no terminated transformation.

Finally, although available in the debugging perspective, the Disconnect and Terminate actions currently have no effect.

Displaying variables values

In the scope of the ATL Debug perspective, the Variables view aims to provide developers with a convenient mean to observe the content of the ATL variables during the execution of a transformation. For this purpose, the Variables view displays all the variables that are visible from the current execution context. Note that the variable self is defined whatever the considered execution context.

In the context of a helper, visible variables correspond to the helper arguments, the local variables introduced by means of the let instruction and the iterator variables that are used in the scope of the collection iterative expressions. The variable self here corresponds to the element in which the context is declared. Except for arguments, the set of visible variables is similar in the scope of an ATL attribute.

During the matching phase of a transformation execution, the variables visible in the context of a matched rule include the source pattern element variable along with the variables and iterators that may be declared in the scope of the source pattern element expression. During the initialization phase, this set of visible variables changes to the rule local variables declared in the rule using section, the source and target pattern element variables and the variables/iterators declared within the executed expressions.

In this example, a breakpoint has been set on the first binding of the target pattern element of rule Author (visible on left column of the Editors view). The Debug view indicates that the operation currently being executed (e.g. the operation __applyAuthor()) corresponds to the initialization phase of the rule Author (the __apply prefix being associated with the rule initialization phase). Going back to the Editors view, it is possible to identify the current instruction which is highlighted in green: it here corresponds to the evaluation of the variable a in the surname binding of the rule target pattern element.

The Variables view makes it possible to navigate the content of the variables that are visible in this context. The variable a corresponds to the source model element currently matched by the rule. The variable p corresponds to the target pattern model element that is currently initialized. Note that, at this stage, since the execution of the surname binding is not completed, the only initialized property of this variable is name. The variable self here points to the ATL module. Finally, the variable link appearing during the transformation initialization phase corresponds to an ATL engine internal variable and could be ignored by the developers. Although not illustrated in the considered example, the Variables view enables to navigate the content of collection variables. It also makes it possible to navigate the source and, at some point, the target model elements using the references defined by these elements.

@@ Line 44: / Line 44: @@
 *access to the source code of installed ATL plugins
-To install ATL from CVS, please refer to the [http://wiki.eclipse.org/ATL/Developer_Guide#Install_ATL_from_CVS Developer Guide].
+To install ATL from CVS, please refer to the [[ATL/Developer_Guide#Install_ATL_from_CVS | Developer Guide]].
 = Overview of the Atlas Transformation Language =

Breadcrumbs

Notice: this Wiki will be going read only early in 2024 and edits will no longer be possible. Please see: https://gitlab.eclipse.org/eclipsefdn/helpdesk/-/wikis/Wiki-shutdown-plan for the plan.

Difference between revisions of "ATL/User Guide"

Revision as of 05:54, 3 June 2009

Contents

Introduction

Installation

Prerequisites

Install ATL

Overview of the Atlas Transformation Language

Examples metamodels

Author metamodel

Person metamodel

Biblio metamodel

ATL module

Structure of an ATL module

Header section

Import section

Helpers

Rules

Module execution modes

Normal execution mode

Refining execution mode

Module execution semantics

Default mode execution semantics

Refining mode execution semantics

ATL Query

Structure of an ATL query

Query execution semantics

ATL Library

The ATL Language

Data types

OclType operations

OclAny operations

The ATL Module data type

Primitive data types

Boolean data type operations

Boolean expressions evaluation

String data type operations

Numerical data type operations

Examples

Collection data types

Operations on collections

Sequence data type operations

Set data type operations

OrderedSet data type operations

Bag data type operations

Iterating over collections

Examples

Enumeration data types

Tuple data type

Map data type

Model element data type

Full name reference to metamodel classes

Examples

ATL Comments

OCL Declarative Expressions

If expression

Let expression

Other expressions

Expressions tips & tricks

ATL Helpers

Helpers

Calling super helpers

Attributes

Limitations

ATL Rules

ATL imperative code

The assignment statement

The if statement

The for statement

Current limitations

Matched Rules

Source pattern

Local variables section

Simple target pattern element

Iterative target pattern element

Imperative block section

Lazy Rules

Unique Lazy Rules