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== Successor to Xpand/Xtend (Work in progress - feedback is highly appreciated) =
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It's time to think about a possible successor to Xpand/Xtend. Although the languages have proven well compared to alternatives there are a number of things which can be better and clearer based on our two years of experience with Xpand and Xtend.
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The main improvements we want to incorporate are:
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=== Imports ===
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The import mechanism should be reworked, so that every import is explicit.
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We won't need any metamodel configuration in the workflow nor in the editors anymore.
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This will not only make the setup simpler but will also improve the performance.
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The syntax would change to something like the following:
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import org:openarchitectureware:Extension; // native import
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import EMF "my/package/model.ecore"; // non-native import
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import UML "my/test.profile.uml" as profile; // non-native import with name space definition
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import Java "my.test.Class"; // non-native import introduces java types and features form my.test.Class
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import XSD "http://www.mywebsite.org/xsd/metamodel.xsd" // non-native import importing the types from an XSD file
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... (think of Hibernate mapping files, Groovy, etc.)
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==== Native import ====
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A native import refers to another extension file imports all public members (types, functions, extensions).
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==== Non-native Import ====
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A non native import starts with an identifier pointing to an installed adapter.
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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
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import keyword defines which adapter to use.
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==== Namespace definition ====
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All members are included without namespace information. If you need a namespace you can explicitely define one per import.
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==== Reexport ====
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The reexport keyword will be supported, so that
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imported stuff (types and functions) will be reexported.
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Example:
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myext1.ext
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  foo() : 'foo';
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myext2.ext
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  reexported import myext1;
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  bar() : 'bar';
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client.ext
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  import myext2;
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  fooBar() : foo()+bar();
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=== Generics ===
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We need full-fledged generics, which can conceptually be copied from Java's generics.
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(complicated) Example:
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  List<M> sort<T extends Comparable<T>, M>(List<M> toSort, (M)=>T closure)
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which can be used like this
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  ['aa','b'].sort(e|e.size); 
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You don't have to deal with this complexity if you don't want to define functions which uses generics ;-)
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=== Closures ===
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We'll have real closures, not the built-in stuff we have now.
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Closure syntax:
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  { parameterList '|' expression-using-parameters-and-scope }
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Where parameter list must be typed, either implicitly or explicitly.
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Example:
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  {
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    String myText := "test";
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    (Attribute)=>Boolean myClosure := {e|e.name==myText};    // e is inferred from the declared type of the assignee
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    myList.select(myClosure);
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  }
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alternatively declare the parameter types explicitly
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  {
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    var myText := "test";
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    var myClosure := {Attribute e|e.name==myText};
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    myList.select(myClosure);
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  }
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  // type of e is inferred from the declaration of the 'select()' function, you don't have to use the curly brackets.
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  myList.select(e|e.name == "test")
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==== Type signatures of functions ====
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The syntax for of a function's type signature looks as follows:
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  (parameterTypes)=>returnType
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Examples:
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  ()=>Object
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  (String)=>Boolean
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  (String, Entity)=>Entity
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Example 2: declaration of higher-order functions using generics :
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  List<T> select<T>(List<T> this, (T)=>Boolean closure) {
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    ...
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  }
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=== Functions ===
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Functions can be invoked either using the functional syntax or using the member syntax (operation like, aka extensions):
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  myFunction(foo, bar) == foo.myFuntion(bar)
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A function is declared as follows:
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  (private|cached|cached) ReturnType? functionName(declaredParameterList) ( guardExpression )? : bodyExpression ;
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Example:
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  private doStuff(Entity this) name!=null :
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      name+"Stuff";
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The detailed semantics of how the polymorphic resolution works (what role guards play here) and is described in the upcoming section.
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or
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  (private|cached) functionName(declaredParameterList) guardExpression blockExpression
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Example:
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  cached makeAbstract(Entity this) {
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      abstract := true;
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      name := 'Abstract'+name;
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      this;
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  }
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Block expressions are explained in their own section.
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==== Polymorphic Resolution with signatures and guards ====
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Usually polymorphism is based on the types of parameters. The same applies for Xtend++.
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In contrast to e.g. Java we use the dynamic types (actual types at runtime) of a given set of parameters in order to find the function which
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best fits (has the most specific declared paramter types).
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Example:
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given the following two functions
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  foo(String x) : "string";
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  foo(Object o) : "object";
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this assertions can be made:
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  foo('S') == "string"
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  foo(34)  == "object"
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  foo((Object)'S') == "string" // would be "object" in Java
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In addition to these concept, commonly known as "multiple dispatch" or "multi method", we introduce guards which can be used to controll the resolution based on the state of a given object not only the type.
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Example:
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  foo(String x) x.length>5 : "long";
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  foo(String x) : "short";
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this assertions can be made:
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  foo('honolulu') == 'long'
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  foo("bar") == 'short'
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The semantics are as follows:
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* First select all possible features (functions and operations), based on the name and the given parameter types.
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* Then order those features by parameter types (best match first).Those functions with the same parameter type should be order so that the ones having a guard defined come first.
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pseudo code
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  for (Feature f : features)  {
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      if (f.hasGuard()) {
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        if (f.guard.evaluate())
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            return f; // return the feature where the guard evaluates to true
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      } else {
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        return f; // return the feature without a guard
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      }
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  }
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  return null; // no invocation
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* if there are features, but the guards evaluate to false, return null:
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The static semantics are straight forward:
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* The guard must be of type boolean.
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====Extensions overwrite semantics====
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Functions and Operations can be overwritten.
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The precedence is based on th order of imports.
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Functions from later declared imports overwrite functions introduced before.
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Local functions overwrite imported functions.
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Consider overwriting the toString() Operation  (which is invoked on String concatenations) for arbitrary meta types.
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This will allow very readable templates.
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==== dynamically scoped extension overwriting ====
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Another thing we want to address is the way one can extend generators provided by third parties (like the one shipped with GMF).
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So far everybody used AOP to "weave" customization and implementation in. The problem is that the generator designer does not really develop for extensibility and every Join Point becomes public API.
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We've been thinking about  a concept called "dynamic extensions" which is a way to register extensions for a specific call graph.
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Example:
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  with(&toString(Entity)) {
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    callGMFCartridge(myEntity);
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  }
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Just to explain: '&toString(Entity) is a literal pointing to the 'toString(Entity)'-function. So one could write '&toString(Entity).evaluate(myEntity)' instead of 'myEntity.toString()' for example. The implementation of the function will be used whenever such a function is invoked within the callGMFCartridge(Entity) function (the third party cartridge).
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In other words  one overwrites the toString() function for Entities for the following block.
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So what you as a generator developer could do is, provide a list of function which can be overwritten. In addition there is a final keyword, which prevents overwriting the corresponding function.
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This won't be a replacement for AOP, but we think that the AOP feature has been missused in order to provide extensibility.
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In addition the extension emchanism was bound to the static context so far which really is a limitation in some situations.
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=== Code blocks ===
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A code block is the replacement for chain expressions ( a-> b-> x) with the additional features:
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* provides variable declarations (Expression returning the assigned value)
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It's something like a pseudo imperative syntax (but still is an expression!).
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Variables are assign-once!
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Example:
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  myExtension(String stuff) {
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    def x := stuff.length();
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    if x>56 then
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        "Foo"
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    else
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        "Bar";
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    endif
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  }
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A code block is itself an expression consisting of a list of expressions.
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It returns the value returned by the last expression.
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It is possible to overwrite the scope.
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Example:
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  doStuff() {
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    def x := "Foo";
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    {
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      def x:= "Bar";
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      x;
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    }
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  }
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will return "Bar"
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=== Object creation expression ===
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We are thinking about a syntax to create model graphs inline.
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We need this not only for model transformations but also for writing model transformation tests.
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Example:
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  new Entity {
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      name := "Person";
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      references += new Reference {
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          name := "someRef"
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          type := anotherEntity
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      }
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  }
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==== Assignment Expressions ====
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They are just another syntax for invoking a setter resp. adder-operation.
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They return the assigned value.
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==== create / cache semantics ====
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The creation expression should replace the "create extension" mechanism from Xtend 1.0.
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A creation of an Object is cached if the type name is suffixed with parenthesis containing any number of arguments.
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The arguments act as a key.
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The scope of the caching is per execution context, which can be reused in several invocations?
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Examples:
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  var paramPerOperationAndName := new Parameter cachedwith op,name {
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      this.name := name;
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      type := aDatatype;
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  }
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  var localSingleton := new Foo cachedwith {
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      stuff := "bla";
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  }
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==== cross referencing ====
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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:
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  var x := new Entity as localRef {
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      // x is not visible here, because the right hand expression has not been evaluated so far.
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      // localRef holds a reference to the created but not yet initialized entity.
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        name := "Person";
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        references += new Reference {
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            name := "partner"
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            type := localRef
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        }
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      }
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=== Operator Overloading ===
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There will be predefined operators which can be used instead of the usual function invocation syntax if there is an operator for a name and a specific number of parameters.
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Some examples:
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  add(Object a, Object b) => a + b
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  subtract(Object a, Object b) => a- b
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  not(Object a) => !a
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The && and || operators are not overwriteable because of there special semantics wrt lazy evaluation.
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=== Overloading accessors ===
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If a function's signature follows the pattern
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  String getFoo()
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it can be invoked using property syntax:
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  this.foo
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If a function's signature follows the pattern
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  void setFoo(String)
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it overwrites the modify of the 'foo' property, hence it will be used within assignments:
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  foo := 'Holla'
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It is also possible to use such functions without having a corresponding property like:
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  getJavaName(Entity this) {
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      if abstract then
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        'Abstract'name
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      else
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        name
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      endif
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  }
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which can be used like:
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myTemplate(Entity this) :»
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  public class «javaName» {
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  }
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«;
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=== Templates ===
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A template is essentially a function returning a String. I always disliked that it is not possible to mix functions and templates within one file. I also find the invocation of templates (EXPAND bla FOR foo) too verbose.
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Example:
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  myTemplate() :»
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    package «packageName»;
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    public class «name» {
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      «foreach (attributes as a)»
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          «if (a.type!=null) then»
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            public «a.type» «a.name»;
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          «endif»
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      «endforeach»
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    }
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  «;
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It's just a string literal with the xpand syntax within. Instead of '«' and '»' one can also use the common literal syntax 'foo' and "bar".
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The FILE statement will be removed.
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Files can be opened through extensions:
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  generateCode(Entity e) :
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    writeToFile(e.JavaFile(),e.toJava());
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==== XString - mutable, streamable, lazy string ====
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We want to come up with a special datatype called XString, which is mutable, streamable and evaluated lazy (on invocation of toString()).
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Because XStrings are mutable and are converted to a string late, it is possible to create a tree structure containing XStrings, where
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you can add XStrings (or normal Strings) later on.
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Example:
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  toJava(Entity this) :"""
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      package «packageName()»;
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      «(def imports:=»
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        import java.util.*;
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      «)»     
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      public class  ...     
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          »assertImported(imports,"java.math.*")«
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      }
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  «;
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  assertImported(XString this, String import) :
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      if !this.contains(import) then
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          this.append("import "import";");
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  ;
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=== if expression ===
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As seen in the previous example, we want an if-expression. using if, else keywords.
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if (predicate) expression (else if (predicate) expression)* (else expression)? endif
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The else part is optional and will return null if not defined.
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Example:
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  // The following expression will return null:
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  if (false) "Holla" endif
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'endif' is optional.
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=== BK added --- To be discussed ===
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==== AOP for Checks ====
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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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SE: We can't modify expressions using AOP. We just can wave an Advice around a function call.
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Shouldn't the designer of a cartridge have this in mind when declaring Join Points to be adviced by clients?
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I think if an advice breaks an existing constraint it breaks the contract of the adviced function.
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=== PF added --- to be discussed ===
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==== Add a FOLDER keyword ====
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In order to create empty directory structures, it would be nice to have a FOLDER keyword.
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Syntax:
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«FOLDER expression»
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SE: I think such things should be implemented using library functions.
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Latest revision as of 08:59, 9 December 2008

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