On 25 Apr 2008, at 15:38, Tom Schrijvers wrote:
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.
By which I gather you mean the interpretation of ":-" as logical
connective "=>" rather than provability "|-"?
What I meant was that when Prolog says "there are no more
solutions", it doesn't know of any more. In realtiy it means "there
no more solutions under the closed world assumption". That means
there could be more solutions if you haven't told Prolog
everything. In this context, there may be more class instances (you
simply haven't told the system yet).
I had no intention to deal with that problem. Just forget what Prolog
says, and when it says "there are no more solutions" think of it as
"no more solutions determined".
(If one interprets ":-" as provability "|-", there needs to be
additional axioms for the object theory, and if the theory is
consistent, it becomes possible to prove that there are no more
solutions.)
My point, though, was to interpret
class a b | a -> b
as a functional dependency b = b(a) rather than as
D a b, D a c ==> b=c
Thus trying to eliminate the use of "=".
I don't follow you here.
I got
?- instance(d, l(A), l(B)).
B = b(A)
?- instance(d, l(A), C).
C = l(b(A))
so pattern-wise C and l(B) are the same. But I do not bother
introduce an explicit operator "=". So at least in this case, if say
D [a] Int will be reject as Int is not of the feom l(_). If one has D
[Int] [Char] and tries to define D [Int] [Float], the leads to trying
to associate the implicit b(Int) with both Char and Float, which
would be rejected.
Hans
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