Restricting Scope

#17 Restricting Scope

Mar 27, 2013 | 8 minutes | Beginners, Scope
Learn how to explicitly and implicitly restrict a scope of an object for a single reference. We will override AbstractDeclarativeScopeProvider reducing the default scope explicitly. Then we will revert changes, add validator and modify content assistant to achieve even better results reducing the default scope implicitly.
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Scope - The region of program source in which an identifier is meaningful (http://en.wiktionary.org/wiki/scope)

The goal

Our DSL contains Greeting and Farewell objects. A greeting is either normal or honest (with the "!" at the end). A Farewell object always needs a corresponding Greeting object. The goal is to allow farewells only for honest greetings. Example:

Hello Eclipse          // normal greeting
Hello XtextCasts!      // honest greeting!

Bye XtextCasts         // OK
Bye Eclipse            // Error

Grammar

grammar org.xtext.example.reducescope.mydsl.MyDsl with org.eclipse.xtext.common.Terminals

generate myDsl "http://www.xtext.org/example/reducescope/mydsl/MyDsl"

Model:
  greetings+=Greeting*
  farewells+=Farewell*
;

Greeting:
  'Hello' name=ID honest?='!'?
;

Farewell:
  'Bye' greeting=[Greeting]
;

Explicit restriction

MyDslScopeProvider.xtend 
package org.xtext.example.reducescope.mydsl.scoping;

import org.eclipse.xtext.scoping.impl.AbstractDeclarativeScopeProvider;
import org.eclipse.xtext.scoping.IScope
import org.xtext.example.reducescope.mydsl.myDsl.Farewell
import org.eclipse.emf.ecore.EReference
import org.eclipse.xtext.scoping.Scopes
import org.xtext.example.reducescope.mydsl.myDsl.Model

public class MyDslScopeProvider extends AbstractDeclarativeScopeProvider {
  def IScope scope_Farewell_greeting(Farewell farewell, EReference eReference) {
    Scopes::scopeFor((farewell.eContainer as Model).greetings.filter [ honest ])
  }
}

Implicit restriction

Adding validation

MyDslJavaValidator.java 
package org.xtext.example.reducescope.mydsl.validation;

import org.eclipse.xtext.validation.Check;
import org.xtext.example.reducescope.mydsl.myDsl.Farewell;
import org.xtext.example.reducescope.mydsl.myDsl.MyDslPackage;

public class MyDslJavaValidator extends AbstractMyDslJavaValidator {

  @Check
  public void checkFarewellGreetings(Farewell farewell) {
    if (farewell.getGreeting() != null && !farewell.getGreeting().isHonest()) {
      error("Only honest greetings (greetings with '!') are allowed here",
            MyDslPackage.Literals.FAREWELL__GREETING);
    }
  }
}

Modifying content assistant

MyDslProposalProvider.xtend 
package org.xtext.example.reducescope.mydsl.ui.contentassist;

import org.xtext.example.reducescope.mydsl.ui.contentassist.AbstractMyDslProposalProvider;
import org.eclipse.emf.ecore.EObject
import org.eclipse.xtext.Assignment
import org.eclipse.xtext.ui.editor.contentassist.ContentAssistContext
import org.eclipse.xtext.ui.editor.contentassist.ICompletionProposalAcceptor
import org.eclipse.xtext.CrossReference
import org.xtext.example.reducescope.mydsl.myDsl.Greeting

public class MyDslProposalProvider extends AbstractMyDslProposalProvider {
        
  override completeFarewell_Greeting(EObject model, Assignment assignment,
                                     ContentAssistContext context, ICompletionProposalAcceptor acceptor) {
    lookupCrossReference(assignment.getTerminal() as CrossReference, 
      context, acceptor, [
        (EObjectOrProxy as Greeting).honest
      ]
    );
  }
}

Version information

  • Eclipse: 4.2 (Juno)
  • Xtext: 2.3