[mercury-users] functional dependencies question

[Ian MacLarty]

Hello,

Why will a program with the following typeclass definitions not compile?

:- typeclass tc1(S, T) <= (S -> T) where [
	pred mem1(S::in, S::out) is det
].

:- typeclass tc2(S, T) <= tc1(S, T) where [
	pred mem2(S::in, S::out) is det
].

:- typeclass tc3(S) <= tc1(S, T) where [].

For tc2 shouldn't the compiler be able to infer that T is functionally 
dependent on S, so mem2 is fine?

For tc3 why does T need to be in tc3's arguments if it is functionally 
dependent on S?

The errors given by the compiler are:

fd.m:016: Error: type variable in superclass constraint is not a 
parameter of this type class: tc1(_1, _3).

fd.m:022: Error: type variable in superclass constraint is not a 
parameter of this type class: tc1(_1, _2).
fd.m:019: In declaration for predicate `fd.mem2/2':
fd.m:019:   error in type class constraints: type variable T occurs in 
the
fd.m:019:   constraints, but is not determined by the predicate's 
argument
fd.m:019:   types.

Is there a fundamental reason for these limitations or are they just 
not yet implemented (or am I just being stupid ;-)

Cheers,

Ian.

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