Eclipse Xtext - Scoping

On the last page we looked at calling a Inheriting from a DSL. On this page we look at scoping in DSLs:

Scoping allows the language designer to decide how non-containment references are handled. Non-containment references are represented in the Ecore model by 'EReference'. 'getScope' is a method injected from IScopeProvider. Given an EObject, and an EReference from it, then getScope will return an IScope containing all the objects that can be reached from it. They may not all be semantically valid.

Example 1 - non-Xbase

When we were looking at how to generate non-containment links (on this page) we used the example from the xtext documentation.

datatype mydata
datatype mydata2
entity mydata {
	Name:mydata
	many mydata:mydata
}
entity mydata2 {
	Name:mydata
	many mydata:mydata
}

Here is the scope hierarchy for that DSM (see that page for the grammar and diagram of the ecore model):

On this diagram: The green boxes represent an instance of IScope. In this example the EReference is looking for the ID='mydata'. In other examples there may be multiple scope trees looking for different IDs. The text in the green boxes show: The type of IScope instance. import normalisers (in first square brackets). all local elements (in second square brackets). The black arrows point to the parent scope.

As you can see on the ecore model (here) there is a non-containment link from 'Feature' to 'Type'. That is from lines like 'Name:mydata' and 'many mydata:mydata' to lines like 'datatype mydata'. These links are formed by setting the ID so that it corresponds to the ID we want to link to.

Scopes are defined for the start of these links, in this case 'Feature' nodes (like 'Name:mydata' and 'many mydata:mydata'). So when we are resolving these links we use the node we are calling from (context) and the reference and we call:

getScope(context,reference)

This returns an instance of IScope (as in the green boxes above).

As we can see in the diagram above, the 'Feature' nodes do not have any matches (second square bracket is empty) so we then go to the parent scope (follow the black arrow) until we find a match. In this case we have to go up to the 'SelectableBasedScope' until we find a match, that is the top level where we have the 'Type' entries as required.

Scoping in XBase Projects

Scoping in Xbase applications is more complicated due to the extra indirection involved with Xbase (as discussed here).

To understand this we need to know the XBase and Types models we need to know where the non-containment links are, that we want to customise the scoping of.

There are lots of EReferences in the model such as the caller of a method references its definition and the user of a class references its definition.

The non-containment EReferences in the JVM Type model are:

notes: references in brackets are arrays.

The non-containment EReferences in the XBase model are:

Example 2 - Xbase

package example6
import example6

class example61 {
  val int myName1 = 1
  val int myName2 = 2

  def int myName1() {
    val int myName1 = myName1
    val int myName2 = myName2
    return myName1
  }

  def int myName2() {
    val int myName1 = myName1
    val int myName2 = myName2
    return myName2
  }
}

So lets look at the first case: 'val int myName1 = myName1' in the class example 61. The 'myName1' needs to be mapped to a value definition somewhere so this must involve an 'EReference' somewhere.

However there are no EReferences in our user(language designer) code so the EReferences must be in the xbase model.

In face this is in a 'JvmParameterizedTypeReference' my code has a 'MyTestFieldImpl' container which holds this.

platform:/resource/mytest/src/example6/example61.dsl#xtextLink_::0.2.0.6.0.3::1::/0) initial=org.eclipse.xtext.xbase.impl.XNumberLiteralImpl@35298697 (value: 1)
reference name=type type=JvmType keys=[] containment=false

The scope hierarchy at this point is: This is the scope hierarchy only from the line 'val int myName1 = myName1'

Something not right here as none of the possible matches seem to be found in the scope?

Example 3 - Inner Class Scoping.

I would like to define the visibility when:

• Referring to an element within a block.
• Referring to an outer class from an inner class.
• Referring to an inner class from an outer class.

The following xbase example uses our usual grammar and jvmModelInferrer files. It uses default validation and scoping handling (no files other than those generated automatically).

The contents of methods are an XBlockExpression and so are handled automatically by xbase. As we can see from the screenshot, there are problems with referring to outer function from inner function.

More about inner classes, using xbase, on this page.

Code

Further Reading

This site:
There is more about cross-referencing, on this page.
For more information about naming go on to this page.

Other sites:
This itemis blog has more information about this topic.
Also this blog by Sebastian Zarnekow.
and this blog by Sven Efftinge
(The last two links are very old and so refer to URL names as opposed to namespaces)
Andreas Graf blog here

EuclideanSpace

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