[Haskell-cafe] Type family fun

[Stefan Holdermans]

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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