Improve "Can't unify" error messages from type functions

Consider this example from a Haskell Cafe thread:

data (:|:) a b = Inl a | Inr b

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

data BiApp a b c = BiApp (a c) (b c)

instance (Ix l, Ix r) => Ix (l :|: r) where
   type IxMap (l :|: r) = BiApp (IxMap l) (IxMap r)
   empty = BiApp empty empty

This elicits the following confusing 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

As Alexander Dunlap responds, the error message is correct, but I can't help feeling that we should try harder to give a better error message. Something like "Since IxMap is a type function, knowing that IxMap l = IxMap i does not require that l = i". Or something.

This ticket is just to make sure we don't forget. The thread is here http://www.haskell.org/pipermail/haskell-cafe/2008-August/046371.html

Simon

Trac metadata

Trac field	Value
Version	6.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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Operating system	Unknown
Architecture	Unknown