David Menendez wrote:
On Thu, Nov 29, 2012 at 7:50 AM, Dmitry Kulagin <dmitry.kula...@gmail.com>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
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