[Haskell-cafe] looking for examples of non-full Functional Dependencies

[Hans Aberg]

On 18 Apr 2008, at 20:04, Martin Sulzmann wrote:

> Let's consider our running example
>
> class D a b | a -> b
> instance D a b => D [a] [b]
>
> which we can write in CHR notation
>
> D a b, D a c ==> b=c    (FD)
>
> D [a] [b] <=> D a b       (Inst)
>
> These rules overlap.

I experimented with translations into GNU Prolog (gprolog). Since "="  
is hard to get working in Prolog unless integrated into unification,  
I tried (using the idea of rewriting unique existence as functions,  
possible if one assumes the axiom of choice):
   class(d, A, b(A)).
   instance(d, l(A), l(B)) :- class(d, A, B).
Then:
   ?- instance(d, l(A), l(B)).
   B = b(A)

   ?- instance(d, l(A), C).
   C = l(b(A))

   ?- instance(d, l(A), l(B)), instance(d, l(A), C).
   B = b(A)
   C = l(b(A))
Though I am not sure about the intended semantics, it does produce  
unique solutions.

   Hans