Multiple constraints

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Description

This article discusses issues which arise with multiple constrained type parameters.

Multiple constraints for generic type parameters

The new ECMA standard for Eiffel introduces multi-constraint generic type parameters.

class C [G -> {A, B }]
end

Class C expects a type parameter that conforms to A and B.

Explanation of the issue

Consider this example inheritance hierarchy together with the following code:

Example: class GENERIC_CLASS [G -> {B, C}]   feature   g: G   example is do g.f -- qualified feature call end   end

Our interest is focused on the qualified feature call g.f of the example feature. With the dynamic binding semantics defined in section 8.16.11 of the ECMA standard, it makes a difference whether the static type of g is A, B or C. An illustration of the differences can be found in the Transposition article.

The standard defines in section 8.12.23 what the base type of such a multiple constrained type parameter is. It is a fictitious class (denoted as a dashed class FICT in the diagram) which inherits from all constraint classes and to which a possible renaming is applied. Dynamic binding requires a clear notion of what the static type of a target (here g) is. To obtain the correct feature, one needs the static and dynamic type of the target at runtime. Normally the base type of the target is taken as the static type. But we cannot take this fictitious base type as our static type, because, as can be seen in the diagram, it is outside of the conformance paths to X and therefore not usable with the current definition of dynamic binding.

The definition of this fictitious type FICT can only be used to clearly define the set of available features to instances of type G. It can not be used to define the semantic of a qualified feature call (like f.a).

Possible solutions

To be able to use the type G one must provide a clear definition which static type to use for a qualified feature call.

Common ancestor

If we have a multi-constraint generic type parameter and a qualified feature call on a target of that type, we define the static type to be the type of the class which introduced the current version of the called feature body. This must be a common ancestor of the constraining classes which contain the feature f (in our case B and C).

In our example we would set the static type of f to A and then execute the qualified feature call.

Properties of this soultion:

• With this soultion the renaming clause is necessary to avoid name clashes.

• All features can be directly applied to g.

• The static type of g may change depending on which feature is called.

The programmer selects a static type by using local variables or the object test

Another totally different idea is to simpliy disallow such calls. One would not be able to do a call like g.f because the feature f has ambiguities regarding dynamic binding.

The extreme case would be to say that there is no feature applicable to instances of G in case where G has mulitple constraints.

The solution is, that the programmer would in an ambiguous case resolve the problem in one of the following ways:

example is
      local
         b: B
      do 
         b := g
         b.f      -- qualified feature call with static type B
      end

What the programmer does is basically a down cast to a single known static type (here B). The way he does it, is over a local variable.

example is
      do 
         if {bg: B | g } then
            bg.f  -- qualified feature call with static type B
         end
      end

This is another way to disambiguate the qualified call.

The idea is alos applicable to name clashes, but it's not a necessity.

Properties of this soultion:

• The renaming clause for multi-constraint types could be removed from the standard because one defines name clashes as ambiguous cases and the user must resolve it by chosing a static type explicitly.

• Not all feature calls can be applied to a target whose type is a multi-constraint generic.

• The definition of the static type of every call remains straight forward.