[mercury-users] typeclass error

[Ian MacLarty]

Hello,

What's wrong with the following program?

:- module tc.

:- interface.

:- import_module io.

:- pred main(io::di, io::uo) is det.

:- implementation.

:- import_module list, int.

main(!IO) :-
         nl(!IO).

:- typeclass tc1(A, C) where [
         func a_to_c(A) = C
].

:- typeclass tc2(B, C) where [
         func b_to_c(B) = C
].

:- func f(A, B) = C <= (tc1(A, C), tc2(B, C)).

f(A, B) =
         ( if a_to_c(A) = b_to_c(B) then
                 a_to_c(A)
         else
                 b_to_c(B)
         ).

I get the following error when I try to compile it:

tc.m:024: In function `tc.f/2':
tc.m:024:   type error: unsatisfied typeclass constraints:
tc.m:024:       `tc.tc1(A, C)'
tc.m:024:       `tc.tc2(B, C)'
tc.m:024: In function `tc.f/2':
tc.m:024:   warning: unresolved polymorphism.
tc.m:024:   The variable with an unbound type was:
tc.m:024:       V_6: C
tc.m:024:   The unbound type variable(s) will be implicitly
tc.m:024:   bound to the builtin type `void'.

Cheers,

Ian.

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