[Haskell-cafe] Could not deduce ... using functional dependencies with GHC7

[Sean Leather]

On Fri, Mar 18, 2011 at 13:35, 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.
>

This doesn't even work in GHC 6.12.3. The search call in the second case,
IdSig, restricts the result to Result Id, which is less polymorphic than
Result id. Either of the following are valid:

instance Search Id (Sig Id) where
  search (SigGen g)   = search g
  search (IdSig i)    = search i

instance (Search id id) => Search id (Sig id) where
  search (SigGen g)   = search g

Sean
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