Revision as of 11:20, 22 September 2008

What is Xtext?

The TMF Xtext project provides a domain-specific language (the grammar language) for description of textual programming languages and domain-specific languages. It is tightly integrated with the Eclipse Modeling Framework (EMF) and leverages the Eclipse Platform in order to provide language-specific tool support. In contrast to common parser generators the grammar language is much simpler but is used to derive much more than just a parser and lexer. From a grammar the following is derived:

• incremental, Antlr3-based parser and lexer
• Ecore-based meta models (optional)
• a serializer, used to serialize instances of such meta models back to a parseable textual representation
• an implementation of the EMF Resource interface (based on the parser and the serializer)
• a full-fledged integration of the language into Eclipse IDE
  • syntax coloring
  • navigation (F3, etc.)
  • code completion
  • outline views
  • code templates
  • folding, etc.

The generated artifacts are wired up through a dependency injection framework, which makes it easy to exchange certain functionality in a non-invasive manner. For example if you don't like the default code assistant implementation, you need to come up with an alternative implementation of the corresponding service and configure it via eclipse extension point.

The Grammar Language

At the heart of Xtext there is the grammar language. The grammar language is defined in itself, of course. The grammar can be found here [[1]].

The grammar leanguage is a DSL carefully designed for description of textual languages, based on ANTLR's LL(*) algorithm. The main idea is to let users describe the concrete syntax, and to automatically derive an in-memory model (semantic model) from that.

First an example

To get an idea of how it works we'll start by implementing a [simple example] introduced by from Martin Fowler. It's mainly about describing state machines used as the (un)lock mechanism of secret compartments.

One of those state machines could look like this:

 events
  doorClosed  D1CL
  drawOpened  D2OP
  lightOn     L1ON
  doorOpened  D1OP
  panelClosed PNCL
 end
 
 resetEvents
  doorOpened
 end
 
 commands
  unlockPanel PNUL
  lockPanel   PNLK
  lockDoor    D1LK
  unlockDoor  D1UL
 end
 
 state idle
  actions {unlockDoor lockPanel}
  doorClosed => active
 end
 
 state active
  drawOpened => waitingForLight
  lightOn    => waitingForDraw
 end
 
 state waitingForLight
  lightOn => unlockedPanel
 end
 
 state waitingForDraw
  drawOpened => unlockedPanel
 end
 
 state unlockedPanel
  actions {unlockPanel lockDoor}
  panelClosed => idle
 end

So we have a bunch of declared events, commands and states. Within states there are references to declared actions, which should be executed when entering such a state. Also there are transitions consisting of a reference to an event and a state. Please read Martin's description is it is not clear enough.

In order to implement this language with Xtext you need to write the following grammer:

 language SecretCompartments
 
 generate secretcompartment "http://www.eclipse.org/secretcompartment"
 
 Statemachine :
  'events'
     events+=Event+
  'end'
  ('resetEvents'
     resetEvents+=[Event]+
  'end')?
  'commands'
     commands+=Command+
  'end'
  states+=State+;
 
 Event :
  name=ID code=ID;
 
 Command :
  name=ID code=ID;
 
 State :
  'state' name=ID
     ('actions' '{' actions+=[Command]+ '}')?
     transitions+=Transition*
  'end';
 
 Transition :
  event=[Event] '=>' state=[State];

In the following the different concepts of the grammar language are explained. We refer to this grammar when useful.

Language Declaration

The first line

language SecretCompartments

declares the name of the language. Xtext leverages Java's classpath mechanism. this means that the name can be any valid Java qualifier. The file name needs to correspond and have the file extension '*.xtext'. So it needs to be "SecretCompartments.xtext" and must be placed in the default package on the Java's class path.

If you want to place it within a package (e.g. 'foo/SecretCompartment.xtext') the first line must read:

language foo.SecretCompartment

The first line can also be used to declare a super language to inherit from. This mechanism is described here.

EPackage declarations

Xtext parsers instantiate Ecore models (aka meta model). An Ecore model basically consists of an EPackage containing EClasses, EDatatypes and EEnums. Xtext can infer Ecore models from a grammar (see #Metamodel Inference) but it is also possible to instantiate existing Ecore models. You can even mix this and use multiple existing ecore models and infer some others from one grammar.

EPackage generation

The easiest way to get started is to let Xtext infer the meta model from your grammar. This is what is done in the secret compartment example. To do so just state:

generate secretcompartment "http://www.eclipse.org/secretcompartment"

Which says: generate an EPackage with name secretcompartment and nsURI "http://www.eclipse.org/secretcompartment" (these are the properties needed to create an EPackage).

EPackage import

If you already have create such an EPackage somehow, you could import it:

import "http://www.eclipse.org/secretcompartment"

Using multiple packages

If you want to use multiple EPackages you need to specify aliases like so:

generate secretcompartment "http://www.eclipse.org/secretcompartment"
import "http://www.eclipse.org/anotherPackage" as another

When referring to a type somewhere in the grammar you need to qualify them using that alias (example "another::CoolType"). We'll see later where such type references occur.

Rules

The parsing is based on ANTLR 3, which is a parser generator framework based on an [LL(*) algorithm]. Basically parsing can be separated in the following phases.

1. lexing
2. parsing
3. model construction
4. linking
5. validation

Lexer Rules

In the first phase a sequence of characters (the text input) is transformed into a sequence of so called tokens. Each token consists of one or more characters and was matched by a particular lexer rule. In the secret compartments example there are no explicitly defined lexer rules, since it uses built-in lexer rules, only (the ID rule).

That rule is defined in the built-in super language (see #Language Inheritance) as follows:

lexer ID : 
  "('^')?('a'..'z'|'A'..'Z'|'_') ('a'..'z'|'A'..'Z'|'_'|'0'..'9')*"; 

It says that a Token ID starts with an optional '^' character followed by a letter ('a'..'z'|'A'..'Z') or underscore ('_') followed by any number of letters, underscores and numbers ('0'..'9'). Note that this declaration is is a black box where you use Antlr syntax directly. Please ignore the optional '^' for the moment.

This is the formal definition of lexer rules:

LexerRule :
  'lexer' name=ID ('returns' type=TypeRef)? ':' body=STRING ';'
;

Return types

A lexer rule returns a value, which defaults to a string (type ecore::EString). However, if you want to have a different type you can specify it. For instance, the built-in lexer rule 'INT' is defined like so:

lexer INT returns ecore::EInt : 
  "('0'..'9')+";

This says, that the lexer rule INT returns instances of ecore::EInt. It is possible to define any kind of data type here, which just need to be an instance of ecore::EDataType. In order to tell the parser how to convert the lexed string to a value of the declared data type, you need to provide your own implementation of 'IValueConverterService'.

Have a look at [org/eclipse/xtext/builtin/conversion/XtextBuiltInConverters.java] to find out how such an implementation looks like.

The implementation needs to be registered as a service (see #Service Framework).

Enum Rules

TODO

String Rules

TODO

Parser Rules

The parser reads in a sequence of tokens produced by the lexer and walks through the parser rules.

Model Construction

Meta-Models

The meta-model of a textual language describes the structure of its abstract syntax trees (AST).

Xtext uses Ecore EPackages to define meta models. Meta models are declared to be either inferred from the grammar #EPackage _generation or imported #EPackage _import. A meta model's declaration can also define an alias name, that is used in other places of the grammar to qualify references to its types. Each language inherits the aliases from its superlanguage, but it can also override these. The default meta model does not have an alias in its declaration. It will contain all types that are referred to without qualifier.

language MyLang

generate MyMetaModel "http://www.mysite.org/myMetaModel" // default meta model
import "http://www.eclipse.org/emf/2002/Ecore" as ecore  // imported meta model with alias

RuleA returns MyType:         // reference to default meta model
  'mytype' name=ID; 
   
RuleB returns ecore::EObject: // reference to imported EPackage
  name=ID;

Meta-Model Inference

By using the generate directive (see #EPackage _generation), Xtext derives the metamodel from the grammar. All elements can occur multiple times, but are generated only once.

Type and Package Generation

Xtext creates

an EPackage

for each generated package declaration. The name of the EPackage will be set to the first parameter, its nsURI to the second parameter. An optional alias allows to distinguish generated EPackages later. Only one generated package declaration without an alias is allowed per grammar.

an EClass

for each return type of a parser rule. If a parser rule does not define a return type, an implicit one with the same name is assumed.

for each type defined in an action or a cross-reference.

an EDatatype

for each return type of a lexer rule.

All EClasses and EDatatypes are added to the EPackage referred to by the alias provided in the type reference they were created from.

Feature and Type Hierarchy Generation

While walking through the grammar, the algorithm keeps track of a set of the currently possible return types to add features to.

• Entering a parser rule the set contains only the return type of the rule.
• Entering a group in an alternative the set is reset to the same state it was in when entering the first group of this alternative.
• Leaving an alternative the set contains the union of all types at the end of each of its groups.
• After an optional element, the set is reset to the same state it was before entering it.
• After a mandatory (non-optional) rule call or mandatory action the set contains only the return type of the called rule or action.
• An optional rule call or optional action does not modify the set.
• A rule call or an action is optional, if its cardinality is '?' or '+'.

While iterating the parser rules and Xtext creates

an EAttribute in each current return type

of type EBoolean for each feature assignment using the '?=' operator. No further EReferences or EAttributes will be generated from this assignment.

for each assignment with the '=' or '+=' operator calling a lexer rule. Its type will be the return type of the called rule.

an EReference in each current return type

for each action. The reference's type will be set to the return type of the current calling rule.

for each assignment with the '=' or '+=' operator in a parser rule calling a parser rule. The EReferences type will be the return type of the called parser rule.

Each EAttribute or EReference takes its name from the assignment/action that caused it. Multiplicities will be 0...1 for assignments with the '=' operator and 0...* for assignments with the '+=' operator.

Furthermore, each type that is added to the currently possible return types automatically inherits from the return type of the current parser rule. You can specify additional common supertypes by means of artificial parser rules, that are never called, e.g.

CommonSuperType:
  SubTypeA | SubTypeB | SubTypeC;

Feature Normalization

As a last step, we examine all generated EClasses and lift up features to supertypes if they are defined by all subtypes.

Error Conditions

The following conditions cause an error

• An EAttribute or EReference has two different types.
• There are an EAttribute and an EReference with the same name in the same EClass.
• There is a cycle in the type hierarchy.
• An new EAttribute, EReference or supertype is added to an imported type.
• An EClass is added to an imported EPackage.
• An undeclared alias is used.
• An imported metamodel cannot be loaded.

Importing existing Meta Models

Language Inheritance

Xtext support language inheritance. By default (implicitly) each language extends a language called *org.eclipse.xtext.builtin.XtextBuiltin* and is defined as follows:

 abstract language org.eclipse.xtext.builtin.XtextBuiltIn_Temp
 
 import "http://www.eclipse.org/emf/2002/Ecore" as ecore;
 
 
 lexer ID : 
   "('^')?('a'..'z'|'A'..'Z'|'_') ('a'..'z'|'A'..'Z'|'_'|'0'..'9')*";
 
 lexer INT returns ecore::EInt : 
   "('0'..'9')+";
 
 lexer STRING : 
   "
   '\"' ( '\\\\' ('b'|'t'|'n'|'f'|'r'|'\\\"'|'\\''|'\\\\') | ~('\\\\'|'\"') )* '\"' | 
   '\\'' ( '\\\\' ('b'|'t'|'n'|'f'|'r'|'\\\"'|'\\''|'\\\\') | ~('\\\\'|'\\'') )* '\\''
   ";
 
 lexer ML_COMMENT : 
   "'/*' ( options {greedy=false;} : . )* '*/' {$channel=HIDDEN;}";
 
 lexer SL_COMMENT : 
   "'//' ~('\\n'|'\\r')* '\\r'? '\\n' {$channel=HIDDEN;}";
 
 lexer WS : 
   "(' '|'\\t'|'\\r'|'\\n')+ {$channel=HIDDEN;}";
 
 lexer ANY_OTHER : 
   ".";

Just ignore the grammar if you don't yet understand it. It basically provides some commonly used lexer rules which can be used in all grammars.

Service Framework

Runtime Architecture

Value Converters

Linking

UI Architecture