Sergey Mechveliani writes: > Second, some compilers say at this: > " > (the instance type must be of form (T a b c) > where T is not a synonym, and a,b,c are distinct type variables) > " > Maybe, they treat Int Int as a repeated variable? Not quite, Int isn't a type variable at all, but an actual type. Basically it's disallowing instance heads more than one type constructor deep. So for example, "instance C (T Int Char)" would also be disallowed. It isn't necessary for this to be anything like this restrictive, of course, but basically it's (a sledgehammer) to enforce (the nut of) non-overlap of instances. SPJ et al proposed relaxing this for [Standard] Haskell [2.0], and ghc will allow the more general cases with -fgla-exts, provided the instances don't _actually_ overlap. (Older versions of ghc (3.00?) allow some overlap, too) Slainte, Alex.
