[Haskell-cafe] ContT and ST stack

[Bas van Dijk]

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.

Bas