* Francesco Mazzoli
At Wed, 02 Jan 2013 12:32:53 +0100, Francesco Mazzoli wrote:
Hi list,
I am a bit puzzled by the behaviour exemplified by this code:
{-# LANGUAGE RankNTypes #-}
one :: (forall a. a -> a) -> b -> b one f = f
two = let f = flip one in f 'x' id three = (flip one :: b -> (forall a. a -> a) -> b) 'x' id four = flip one 'x' id
Try to guess if this code typechecks, and if not what’s the error.
While `two' and `three' are fine, GHC (7.4.1 and 7.6.1) complains about `four':
Line 8: 1 error(s), 0 warning(s)
Couldn't match expected type `forall a. a -> a' with actual type `a0 -> a0' In the third argument of `flip', namely `id' In the expression: flip one 'x' id In an equation for `four': four = flip one 'x' id
So for some reason the quantified variable in `id' gets instantiated before it should, and I have no idea why.
Any ideas?
Francesco
OK, I should have looked at the manual first. From http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extension...: “For a lambda-bound or case-bound variable, x, either the programmer provides an explicit polymorphic type for x, or GHC's type inference will assume that x's type has no foralls in it.”. So there is a difference between let-bound things and the rest.
I don't see how this is relevant. GHC correctly infers the type of "flip one 'x'": *Main> :t flip one 'x' flip one 'x' :: (forall a. a -> a) -> Char But then somehow it fails to apply this to id. And there are no bound variables here that we should need to annotate. Roman