Chris,
In the inferred type, there should be IxMap l instead of IxMap i,
does anybody know what I'm doing wrong?
Your calls to empty are just ambiguous.
Let's say I want to get a hold of an empty map for A :|: B for some
types A and B. And let's say that you've instance for A hanging around
that specifies type IxMap A = C. Now our call to empty for A :|: B
delegates to empty to get the left map. Clearly you expect it to call
to the instance for A, but any type D with IxMap D = C would do.
Does that make sense?
Cheers,
Stefan
On Aug 23, 2008, at 4:55 PM, Chris Eidhof wrote:
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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