[Haskell-cafe] List comprehensions and impredicative rank-N types

[Vladimir Reshetnikov]

Hi,

Consider the following definitions:

-----------------------------------------------------------
{-# LANGUAGE RankNTypes, ImpredicativeTypes #-}

foo :: [forall a. [a] -> [a]]
foo = [reverse]

bar :: [a -> b] -> a -> b
bar fs = head fs
-----------------------------------------------------------

According to the Haskell Report, [f | f <- foo] translates to (let ok
f = [f]; ok _ = [] in concatMap ok foo), right?

So, I wonder why (bar [f | f <- foo]) typechecks, but (bar (let ok f =
[f]; ok _ = [] in concatMap ok foo)) and (bar foo) do not typecheck?

Thanks,
Vladimir