[Haskell-cafe] Type family fun

[Ryan Ingram]

On Sun, Aug 24, 2008 at 1:44 AM, Stefan Holdermans <stefan at cs.uu.nl> wrote:
] Your calls to empty are just ambiguous.

Now, you are probably wondering how to fix it.  Here's two different solutions:

> {-# LANGUAGE TypeFamilies, TypeOperators, ScopedTypeVariables #-}
> module Ix where

The first solution still uses type families, but "empty" takes a
parameter so that the which instance to use can be chosen
unambiguously.

> class Ix i where
>     type IxMap i :: * -> *
>     empty :: i -> IxMap i [Int]

> -- uses ScopedTypeVariables
> instance (Ix left, Ix right) => Ix (left :|: right) where
>    type IxMap (left :|: right) = BiApp (IxMap left) (IxMap right)
>    empty _ = BiApp (empty (undefined :: left)) (empty (undefined :: right))

The second solution uses data families instead, because no such
ambiguity can exist.

> class IxD i where
>     data IxMapD i :: * -> *
>     emptyD :: IxMapD i [Int]

> instance (IxD left, IxD right) => IxD (left :|: right) where
>     data IxMapD (left :|: right) a = BiAppD (IxMapD left a) (IxMapD right a)
>     emptyD = BiAppD emptyD emptyD

  -- ryan

> data (:|:) a b = Inl a | Inr b
> data BiApp a b c = BiApp (a c) (b c)