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Revision as of 17:43, 13 November 2007 by (Talk | contribs) (Definition of Types (later))

Collection of features (rough)

In Version 5 we want to improve some of the Xtend language concepts and features. Codename is Xtend++ :


The import mechanism should be reworked, so that every import is explicit. We won't need any metamodel configuration in the workflow nor in the editors anymore. This will not only make the setup simpler but will also improve the performance.

The syntax would change to something like the following:

import org:openarchitectureware:Extension; // native import
import EMF "my/package/model.ecore"; // non-native import
import UML "my/test.profile.uml" as profile; // non-native import with name space definition
import Java "my.test.Class"; // non-native import introduces java types and features form my.test.Class
import XSD "" // non-native import importing the types from an XSD file
... (think of Hibernate mapping files, Groovy, etc.)

Native import

A native import refers to another extension file imports all public members (types, functions, extensions).

Non-native Import

A non native import starts with an identifier pointing to an installed adapter. The adapter is responsible for loading and converting the type information from the given string. The syntax in the string is defined by the adapter. The token directly after the import keyword defines which adapter to use.

Namespace definition

All members are included without namespace information. If you need a namespace you can explicitely define one per import.


The reexport keyword will be supported.

MV: What will it do?
BK: Reexport an import to "downstream" extensions


We need full-fledged generics, which can conceptually be copied from Java's generics. Maybe we can leave out some of the more advanced capabilities?


We'll have real closures, not the built-in stuff we have now. Closure syntax:

  parameterList '|' expression-using-parameters-and-scope

Where parameter list must be typed, either implicitly or explicitly.


  // type of e is inferred from the declaration of the 'select()' function| == "test")


   String myText := "test";
   (Attribute)=>Boolean myClosure := e|;     // e is inferred from the declared type of the assignee;

alternatively declare the parameter types explicitly

   var myText := "test";
   var myClosure := Attribute e|;;

Declaring Closure types

The syntax for of a closure type is

MV: actually what you do here is not declare closure types, but rather function signatures. Closures are more or less function literals. So there are several things:

  • function signatures
  • anonymous functions (closures)
  • closures assigned to a variable (which makes them named functions defined via a closure)
  • and normal functions

stuff like select(...) does not expect a closure, rather it is typed with a function literal (something like (Attribute)=>boolean). You can pass either

  • a literal closure
  • a refernce to a named closure
  • or a reference to a function


  (String, Entity)=>Entity

Example 2: declaration of higher-order functions using generics :

 List<T> select<T>(List<T> this, (T)=>Boolean closure) {


In the first version there will only be functions (aka. extensions). Functions can be invoked either using the functional syntax or using the member syntax (operation like):

  myFunction(foo, bar) == foo.myFuntion(bar)

A function is declared as follows:

  (private|cached) ReturnType? functionName(declaredParameterList) guardExpression? : bodyExpression ;


  private doStuff(Entity this) name!=null :

MV: What happens if the guard is false? Not executed? Another one of the same name is executed?
> BK: See next section. I'd prefer to have one expression with NO guard which is used as default if all the others evaluate to false. I think everything else will be the hell for the tooling!


  (private|cached) functionName(declaredParameterList) guardExpression blockExpression

MV: What is a block expression?
> BK: just an expression


  cached makeAbstract(Entity this) {
     abstract := true;
     name := 'Abstract'+name;
     return this;

Polymorphic Guards

The guards are used for polymorhpic resolution of a called extension.+

MV: Polymorphic is maybe the wrong term, because it is associated with subtyping.


context Entity {
   String javaName() isAbstract : "Abstract"+name;
   String javaName() : name;

The dynamic semantics are:

  • First select all possible features (functions and operations), based on the name and the types.
  • Then order those features by parameter types (best match first).
  • For each feature
  • evaluate the guard
  • if it doesn't evaluate to true skip it
  • if it evaluates to true
  • make sure that there is no second feature with the same signature where the guard evaluates to true
  • if there is no such feature, we've found our feature -> success!
  • else throw new Exception("Unambigous feature ... more than one feature ... bla")
  • if there are features, but the guards evaluate to false, return null:

MV: Why null? Why not fail?
> BK: see my comment above

  • if there is no feature, do "feature not found" (i.e. raising exception or invoking 'featureNotFound' operation)

The static semantics are straight forward: The guard must be of type boolean.

MV: do we want to do static verification that all cases are covered?
> BK: see my comment above

Extensions overwrite semantics

Extensions with the same signature will overwrite Operations. Consider overwriting the toString() Operation (which is invoked on String concatenations) for arbitrary meta types. This will allow very readable templates.

Code blocks

A code block is the replacement for chain expressions ( a-> b-> x) with the additional features:

  • provides variable declarations (Expression returning the assigned value)
  • support early exit using the 'return' keyword, which forces outer code blocks to exit themselfes.

It's something like a pseudo imperative syntax (but still is an expression!).

MV: Variables are assign-once, right?


 myExtension(String stuff) {
    var x := stuff.length();
    if (x>56)
       return "Foo";
    else {
       return "Bar";

A code block is itself an expression consisting of a list of expressions. It returns the value returned by the first executed "return-expression", or the value of the last expression.

It is possible to overwrite the scope. Example:

doStuff() {

  var x := "Foo";
     var x:= "Bar";
     return x;


will return "Bar"

Object creation expression

We are thinking about a syntax to create model graphs inline. We need this not only for model transformations but also for writing model transformation tests.


  new Entity {
      name := "Person";
      references += new Reference {
         name := "someRef"
         type := anotherEntity

Assignment Expressions

They are just another syntax for invoking a setter resp. adder-operation (which will be removed).

MV: Great!
> BK: I'd prefer not to remove them. (Compatibility) Do we intent to support operator overloading? Then, this could just be a function with two arguments

They return the assigned value.

create / cache semantics

The creation expression should replace the "create extension" mechanism from Xtend 1.0. A creation of an Object is cached if the type name is suffixed with parenthesis containing any number of arguments. The arguments act as a key.

MV: What is the scope of the caching? One Xtend Component invocation?
> BK: I don't think so. It is the creation plus the block for assignments. Is removing the create keyword a good idea? I like it the way it is. E.g. by default these kind of extensions always return the created object. in the new syntax, you'd have to do that manually or put everything in the init-block. I can see the benefits. Nevertheless I think the old one is useful as well. BTW: new Foo will still be valid, right? Without caching!


  var paramPerOperationAndName := new Parameter(op,name) { := name;
     type := aDatatype;
  var localSingleton := new Foo() {
     stuff := "bla";

MV: The syntax with the arguments in the params looks quite a bit like a constructor. This is confusing. Can we use something else? like using < and >, for example?

BK: what happens to the parameters? Nothing, right?

cross referencing

We need a way to specify cross references within a declared tree. The problem is that we need a reference to a created type after it has been created and before it will be initialized. This can be accomplished by adding a special assignment construct:

BK: Why do we need that? From a user's point of view, this is useless, right?

  var x := new localRef:=Entity {
     // x is not visible here, because the right hand expression has not been evaluated so far.
     // localRef holds a reference to the created but not yet initialized entity.
        name := "Person";
        references += new Reference {
           name := "partner"
           type := localRef

MV: hm, the syntax with the two := is not very nice. How about:

  var x := new Entity as localref {

XString (Template syntax)

We want to come up with a special datatype calle XString, which has a special literal syntax (like Xpand template syntax) and is mutable and streamable.


  toJava(Entity this) :"""
    package «packageName()»;
     public class «name» {
       «FOREACH attributes AS a» 
        public «a.type» «»;

It's just a string literal with the xpand syntax within. The terminals '«' and '»' should be configurable (or there should be an alternative at least).

The FILE statement will be removed. Files can be opened through extensions:

 generateCode(Entity e) :

MV: Nice!!


Because XStrings are mutable and are converted to a string late, it is possible to create a tree structure containing XStrings, where you can add XStrings (or normal Strings) later on.


  toJava(Entity this) :"""
     package «packageName()»;
  cached imports(Entity this) :"""
     import java.util.*;

... to be continued (and cleaned up;-))

if expression

As seen in the previous example, we want an if-expression. using if, else keywords.

if (predicate) expression (else if (predicate) expression)* (else expression)?

The else part is optional and will return null if not defined.


  // The following expression will return null:
  if (false) "Holla"

Declarative constraints

The language check will be integrated into Xtend. That is you can define checks and extensions in the same file:

allEntities(emf::EObject obj) : eRootContainer.eAllContents.typeSelect(Entity);

context Entity warning "name "+name+" should start with an uppercase letter" : 
context Entity error "duplicate name "+name : 

of course you can use everything we have defined before in checks, too. Example :

context Entity error "duplicate name "+name {
      var entities := eRootContainer.eAllContents.typeSelect(Entity);

Definition of Types (later)

So far we couldn't define Types within Xtend but had to define them using other techniques (ecore, Java, UML-profile, etc.). Defining tyoes within Xtend would be a great feature. Because it is much simpler and faster to write them in text. In addition we could define Type with logic (operations). A syntax could look like this:

type Entity extends Named {
  // simple attributes
  String name;
  Boolean isAbstract {
     private set(aValue)
     get : name.startsWith("Abstract");
  // references
  Set<Entity> superTypes;
  Set<Features>* features; // asterisk means containment
  Set<Reference>* references subsets features;
  // operations
  doStuff(String x) : x+name;
  doMoreStuff(String x) {
     name := x;
     features += var f := new Feature{

MV: I agree that this is not the most urgent feature.

BK added --- To be discussed

Add private keyword to XPand

This is useful for marking Definitions in Xpand-Files as Private-API, especially useful when using AOP-features. Otherwise the whole generator is public API which is not intended in most cases

AOP for Checks

When modifying an expression using AOP, there might be the situation that a check is no longer valid. It has to be modified as well

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