On Fri, 25 Apr 2008, Hans Aberg wrote:
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.
Prolog works under the assumption of a closed world. That's contrary to
the open world view of regular type classes. So these aren't the intended
semantics.
Tom
--
Tom Schrijvers
Department of Computer Science
K.U. Leuven
Celestijnenlaan 200A
B-3001 Heverlee
Belgium
tel: +32 16 327544
e-mail: [EMAIL PROTECTED]
url: http://www.cs.kuleuven.be/~toms/
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