Re: [Haskell-cafe] Can not use ST monad with polymorphic function

David Menendez wrote:

On Thu, Nov 29, 2012 at 7:50 AM, Dmitry Kulagin wrote:

...

Thank you, MigMit!

If I replace your type FoldSTVoid with: data FoldMVoid = FoldMVoid {runFold :: Monad m => (Int -> m ()) -> m ()}

then everything works magically with any monad! That is exactly what I wanted, though I still do not quite understand why wrapping the type solves the problem

Short answer: It's because GHC's type system is predicative.

Basically, quantified types can't be given as arguments to type constructors (other than ->, which is its own thing). I'm not entirely sure why, but it apparently makes the type system very complicated from a theoretical standpoint. By wrapping the quantified type in a newtype, the argument to IO becomes simple enough not to cause problems.

GHC has an extension -XImpredicativeTypes that lifts this restriction, but in my experience, it doesn't work very well. A newtype data Foo = Foo { bar :: forall a . baz a } usually works best. Best regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com