oleg@pobox.com writes:
There is a great temptation to read the following declarations
class Pred a b instance (Ord b, Eq b) => Pred [Int] b instance Pred Bool Bool
as a Prolog program:
pred([int],B) <- ord(B),eq(B). pred(bool,bool).
(In Prolog, the names of variables are capitalized and the names of constants begin with a lower-case letter. In Haskell type expressions, it's the other way around).
Why not simply say that type classes have a natural interpretation in terms of Constraint Handling Rules (CHRs). For example, the above instances translate to rule Pred [Int] <==> Ord b, Eq b rule Pred Bool Bool <==> True CHRs specify rewrite rules among constraints (from left to right). A CHRs fires if we find a constraint which matches the left hand side (compare this to Prolog where we use unification). Why CHRs are great for reasoning about type classes is well-documented here: http://www.comp.nus.edu.sg/~sulzmann/research/node4.html Martin