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Revision as of 07:20, 13 November 2007 by (Talk | contribs) (Object construction pattern)

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 overworked, so that every import is explicite. 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.

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.

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). 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.


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

= cross referencing

We need a way to specify crossreferences within a deklared 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:

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

Extensions vs. Functions

I (Sven) would like to discuss if it wouldn't be a good idea to explicitely separate between functions and extensions.

I think the author of an extension can decide how it should be invoked (
my.ext() vs. ext(my)

This would make the syntax more strict, so we can provide better error messages. In addition I think it would make extension files more readable. The syntax could look like the following:

myFunction(String stuff) : 

context Entity {
   javaName() : name.toFirstUpper();
   allAttributes() : ....;
   anotherExtension(String name) : ...; 

That is functions in a context [Type] {}-Block are extension - everything else is a function.

Code blocks

I (Sven) would like to discuss a ruby-like code block syntax. A code block is similar to a chain expression ( a-> b-> x) with the additional features:

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

It's something like a pseudo imperative syntax.


 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.

We should make the colon and semicolon optional so that

myExt() { "stuff"; }


myExt() : { "stuff"; };

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"

Integration of Check

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" : 
   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

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{

Template syntax

We want to come up with a simple template syntax integrated into Xtend. Example:

 context Entity {
   toJava() :
    »package «packageName()»;
     public class «name» {
        public «a.type» «»;

It's just a string literal with the xpand terminals '«' and '»'. In addition if you use these type of string literals, string concatenation is implicit. Under the hood we might come up with a special string type (for performance reasons), but it should be compatible to the oaw String type and therefore transparent to the user.

Files are opened thorugh extensions:

 generateCode(Entity e) :

Something like this...

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.

Optional parentheses for operations, functions, extensions without parameters

We should discuss if we want to make empty parentheses on invocations of operations optional. So that :




Polymorphic Guards

I (Sven) would like to discuss if we want to have the ability to specify guards on functions, extensions and operations. Example:

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

In addition I would like to use guards in a polymorphic context:

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

The dynamic semantics are:

  • First select all possible features (functions or extensions 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:
  • 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.

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

Example:| == "test")


   String myText := "test";
   (Attribute)=>Boolean myClosure := e|;;

alternatively make use of type inference

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

declaration of select :

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

Xpand Container

We'll have a possibility to add a string to a previously defined "outlet"

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