* Francesco Mazzoli <[email protected]> [2013-01-02 13:04:36+0100] > 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-extensions.html#id623016>: > “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 _______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
