[Haskell-cafe] Type family fun
chris at eidhof.nl
Sat Aug 23 10:55:23 EDT 2008
I was playing around with type families, and I have a strange problem.
Suppose we have an alternative to an Either datatype:
> data (:|:) a b = Inl a | Inr b
and a class Ix:
> class Ix i where
> type IxMap i :: * -> *
> empty :: IxMap i [Int]
Now I want to give an instance for (a :|: b):
> instance (Ix l, Ix r) => Ix (l :|: r) where
> type IxMap (l :|: r) = BiApp (IxMap l) (IxMap r)
> empty = BiApp empty empty
BiApp is defined as following:
> data BiApp a b c = BiApp (a c) (b c)
However, it looks like the recursive calls to empty can't be unified,
I get the following error message:
Couldn't match expected type `IxMap l'
against inferred type `IxMap i'
Expected type: IxMap (l :|: r) [Int]
Inferred type: BiApp (IxMap i) (IxMap i1) [Int]
In the expression: BiApp empty empty
In the definition of `empty': empty = BiApp empty empty
In the inferred type, there should be IxMap l instead of IxMap i, does
anybody know what I'm doing wrong?
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