On 2006-04-13, Martin Sulzmann
I believe that GHC's overlapping instance extensions effectively uses inequalities.
Why do you think that 'inequalities' model 'best-fit'?
instance C Int -- (1) instance C a -- (2)
under a 'best-fit' instance reduction strategy we would resolve C a by using (2).
'best-fit' should be very easy to implement. Simply order instances (resulting CHRs) in an appropriate 'best-fit' order.
In case of
instance C Int instance a =!=Int | C a (2')
we can't reduce C a (because we can't satisfy a=!=Int)
Notice that (2') translates to
rule C a | a =!=Int <==> True
I think it's better to write a =!=Int not as part of the instance context but write it as a guard constraint.
I don't think there's any issue for an implementation (either using 'best-fit' or explicit inequalities). The hard part is to establish inference properties such as completeness etc.
This best-fit is essentially what people doing multi-method dispatch want. It turns out to not be as trivial as one would hope. -- Aaron Denney -><-