[mercury-users] Existential Types: Passing over typeclass of enclosed types

[Jörg Roman Rudnick]

Dear all,

in his Ph.D. thesis,

David Jeffery:
    Expressive Type Systems for Logic Programming Languages
    University of Melbourne; February, 2002,

David Jeffery in p. 82 ff. sheds an impression a typeclass of an 
enclosed type may be passed over to its existential type wrapper, i.e.

---------------------------------
:-typeclass x_y_point(T) where [
    pred get_x(T::in, float::out) is det,
    pred get_y(T::in, float::out) is det,
    pred set_x(T::in, float::in, T::out) is det,
    pred set_y(T::in, float::in, T::out) is det
].

:-type x_y_point --->
    some [T] x_y_point(T) => x_y_point(T).

:-instance x_y_point(x_y_point).

:-instance x_y_point(x_y_point) where [
    ( get_x(x_y_point(P), X) :-
            get_x(P, X)
    ),
    ( get_y(x_y_point(P), Y) :-
            get_y(P, Y)
    ),
    ( set_x(x_y_point(P0), X, 'new x_y_point'(P)) :-
            set_x(P0, X, P)
    ),
    ( set_y(x_y_point(P0), Y, 'new x_y_point'(P)) :-
            set_y(P0, Y, P)
    )
].
---------------------------------

In consideration inhowfar to apply existential types, I discovered this 
does not seem so for

    pred add(T::in, T::in, T::out) is det,
...
    ( add(x_y_point(P1), x_y_point(P2), 'new x_y_point'(P)) :-
            add(P1, P2, P)
    ),

with the consequence that one can't use more than a single parameter in 
in-mode - otherwise, a type clash is reported, as the compiler expects 
the exact type instances to be different.

Now my question:

As

(1) it seems the very common case that such params actually do share the 
same type_desc, and

(2) being able to use several existentially typed in-mode params is most 
useful (or even critical) for application,

is it possible to add an explicit assertion that identical types are used?



Thank you in advance,

          Nick











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