合金中的类型错误

Question

I have an Alloy specification to represent a subset of java programming language. Below we have some part of this model:

  abstract sig Type {}

  one sig void_ extends Type {}

  abstract sig PrimitiveType extends Type {}

  one sig Int_, Long_ extends PrimitiveType {}

  sig Method {
       id : one MethodId,
       param: lone Type,
       acc: lone Accessibility,
       return: one Type,
       b: one Body
    }{
     (return=void_) => ( 
        ( (b=ConstructorMethodInvocation) => (b.cmethodInvoked).return = void_) ||
        ( (b=MethodInvocation) => ((b.id_methodInvoked).return = void_) ) 
                       )
    }

    abstract sig Body {}

    sig MethodInvocation extends Body {
        id_methodInvoked : one Method,
        q: lone Qualifier
    }

    sig ConstructorMethodInvocation extends Body {
        id_Class : one Class,
        cmethodInvoked: one Method
    }{
        this.@cmethodInvoked.acc != private_
        this.@cmethodInvoked in ((this.@id_Class).*extend).methods
    }

In this code, there is a type error in Method signature, that is:

This cannot be a legal relational join where
left hand side is this . (this/Method <: b) .
(this/ConstructorMethodInvocation <: cmethodInvoked) (type =
{this/Method})
right hand side is this . (this/Method <: return) (type =
{this/Type})

And i am not understanding why.

Thanks in advance,

Answer 1

The problem is that "return" is implicitely scoped and thus represents the result of "this.return", ie "return" is already a "Type", not a relation from "Method" to "Type".

A possible solution is to avoid the implicit scoping by using a separate "fact" section and explicitly quantify over all "Methods":

fact {
  all m: Method | (m.return = void_ .....
}