On Friday 18 March 2011 13:35:22, JP Moresmau wrote:
These are GHC types, but here is a self-contained example: {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances #-}
data Id=Id String
data Result id =ResultId Id | ResultGen id
data Sig id=IdSig Id | SigGen id
class Search id a | a -> id where search :: a -> Result id
instance Search Id Id where search i = ResultId i
instance (Search id id) => Search id (Sig id) where search (SigGen g) = search g search (IdSig i) = search i
The last line fails. I don't understand why this doesn't compile.
In (IdSig i), i has type Id, hence search i :: Result Id but you want something of type `Result id'. Fortunately it's easy to transform, since search i is a ResultId, so instance (Search id id) => Search id (Sig id) where search (SigGen g) = search g search (IdSig i) = case search i of ResultId y -> ResultId y _ -> error "foo" compiles (and probably does what you want).