Re: [Haskell-cafe] instance deduction and phantom types

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 wrote:

Also, you may find it helpful to add

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{-# LANGUAGE KindSignatures #-}

to the top of your program, and start annotating your class and instance declarations for clarity:

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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 wrote:

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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

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data Patch x y = ...

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

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However, the instance declaration:

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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:

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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