Re: [Haskell-cafe] Type family fun

24 Aug 2008


      On Sun, Aug 24, 2008 at 1:44 AM, Stefan Holdermans  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)

Re: [Haskell-cafe] Type family fun

Ryan Ingram