[Haskell-cafe] instance deduction and phantom types

[Jason Dagit]

Thank you for the quick response.  It's very helpful and makes perfect sense
now.

On Thu, Aug 7, 2008 at 7:58 PM, Ryan Ingram <ryani.spam at gmail.com> wrote:

> Also, you may find it helpful to add
>
> > {-# LANGUAGE KindSignatures #-}
>
> to the top of your program, and start annotating your class and
> instance declarations for clarity:
>
> > class Conflict (p :: * -> * -> *) where
> >    hasConflict :: p x y -> Bool
>

Ah, yes we do that sometimes, but not with consistency.


>
> On Thu, Aug 7, 2008 at 7:51 PM, Ryan Ingram <ryani.spam at gmail.com> wrote:
> > I'm just guessing from this email; I can't see your code.  Let me know
> > if I go wrong anywhere.
> >
> > It looks like you expect your patches to be some type (p :: * -> * ->
> > *); that is, a type like
> >
> >> data Patch x y = ...
>

Yes, that's right and sorry for not including Patch.


>
> >
> > However, the instance declaration:
> >
> >> instance Conflict (p x y) => Conflict (PatchInfoAnd (p x y))
> >
> > is referring to ANY types
> >   p :: * -> * -> * -> * -> *
> >   x :: *
> >   y :: *
> > so that (p x y :: * -> * -> *); that is, a type like this:
> >
> >> data PatchBroken a b x y = ...
> >
> > in this case an instance for Conflict (PatchBroken Int Bool) would
> > give instances for Conflict (PatchInfoAnd (PatchBroken Int Bool)).
>

That makes perfect sense.  So, by including x and y in the instance I was
dramatically changing the kind inference.  That didn't occur to me earlier,
but it does make sense.

Thanks!
Jason
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