[Haskell-cafe] ContT and ST stack

[Max Bolingbroke]

On 10 March 2011 17:55, Bas van Dijk <v.dijk.bas at gmail.com> wrote:
> On 10 March 2011 18:24, Yves Parès <limestrael at gmail.com> wrote:
>> Why has the operator (.) troubles with a type like (forall s. ST s a)?
>>
>> Why can't it match the type 'b' in (.) definition?
>
> As explained by the email from SPJ that I linked to, instantiating a
> type variable (like 'b') with a polymorphic type (like 'forall s. ST s
> a' ) is called impredicative polymorphism. Since GHC-7 this is not
> supported any more because it was to complicated.

AFAIK this decision was reversed because SPJ found a simple way to
support them. Indeed, they work fine in 7.0.2 and generate warnings.
Try it out:

{{{
{-# LANGUAGE ImpredicativeTypes #-}
module Impred where

f :: Maybe (forall a. [a] -> [a]) -> Maybe ([Int], [Char])
f (Just g) = Just (g [3], g "hello")
f Nothing  = Nothing
}}}

Unfortunately, the latest user guide still reflects the old situation:
http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html

Cheers,
Max