On Sat, Aug 23, 2008 at 7:55 AM, Chris Eidhof
Hey all,
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?
Thanks,
-chris
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Hi, I'm not very familiar with type families, but shouldn't BiApp be defined as
data BiApp a b c = BiApp (a b) (a c)
since you're applying it as BiApp (IxMap l) (IxMap r)? Alex