[Haskell-cafe] ContT and ST stack

[Bas van Dijk]

On 10 March 2011 14:47, Daniel Fischer <daniel.is.fischer at googlemail.com> wrote:
> If memory serves correctly, it's impredicative polymorphism.

Indeed. For example the following also doesn't type check in GHC-7:

foo :: (forall s. ST s a) -> a
foo st = ($) runST st

Surprisingly the following does:

foo :: (forall s. ST s a) -> a
foo st = runST $ st

Because GHC contains a special rule for infix $. Also see:

http://article.gmane.org/gmane.comp.lang.haskell.glasgow.user/19152

Bas