Am Donnerstag 04 März 2010 02:39:30 schrieb Tyson Whitehead:
On March 3, 2010 18:35:26 Daniel Fischer wrote:
Because:
instance Applicative ((->) a) -- Defined in Control.Applicative
so, from the instance Z (a -> b), with b == c -> d, we have an
instance Z (a -> (b -> c))
and from instance Z (m (u -> v)), we have, with m == ((->) x), an
instance Z (x -> (u -> v))
Thanks Daniel,
That makes sense. Strangely enough though, I had actually originally tried it with my own Applicative class just in case I was being tripped up by something like the (->) instance you pointed out, and it still didn't work.
Well, GHC takes only the class head into account for instance selection, and u -> (v -> w) matches both, a -> b -- (a == u, b == v -> w) and m (c -> d) -- (m == ((->) u), c == v, d == w), so there's the overlap without any other type classes involved. And since u -> (v -> w) matches both instance heads, type W (u -> (v -> w)) = u -> (v -> w) and type W (((->) u) (v -> w)) = (u -> v) -> (u -> w) are indeed conflicting, so you can't even use OverlappingInstances etc. to make it work.
That is
{-# LANGUAGE FlexibleInstances, TypeFamilies #-}
newtype I a = I a
class A t where ap :: t (a -> b) -> t a -> t b
class Z t where type W t z :: t -> W t
instance A I where ap (I f) (I x) = I $ f x
instance Z (a -> b) where type W (a -> b) = a -> b z = id
instance A t => Z (t (a -> b)) where type W (t (a -> b)) = t a -> t b z = ap
also gives me
Temp.hs:17:9: Conflicting family instance declarations: type instance W (a -> b) -- Defined at Temp.hs:17:9 type instance W (t (a -> b)) -- Defined at Temp.hs:21:9 Failed, modules loaded: none.
Is the compiler somehow anticipating that I could add an instance for (->) to A and thus be back to the Applicative class situation?
The compiler works on an open-world assumption, if the kinds match, there could be an instance defined somewhere.
Thanks! -Tyson
PS: I asked this here because type classes is a GHC issue, would the haskell- cafe list been a more appropriate place?
Either is fine. Cheers, Daniel