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
Dmitry
On Thu, Nov 29, 2012 at 12:01 AM, MigMit <miguelim...@yandex.ru> wrote:
> Yes, monomorphism. "do" binding requires your fold'' to be of some
> monomorphic type, but runST requires some polymorphism.
>
> If you want, you can use special type like that:
>
> data FoldSTVoid = FoldSTVoid {runFold :: forall a. (Int -> ST a ()) -> ST
> a ()}
>
> fold :: Monad m => (Int -> m ()) -> m ()
> fold f = mapM_ f [0..20]
>
> selectFold :: String -> IO FoldSTVoid -- ((Int -> m ()) -> m ())
> selectFold method = do
> -- in real program I'd like to choose between
> -- different fold methods, based on some IO context
> return $ FoldSTVoid fold
>
> useFold :: FoldSTVoid -> ST a ()
> useFold fold' = runFold fold' f
> where f _ = return () -- some trivial iterator
>
> main = do
> fold'' <- selectFold "some-method-id"
> print $ runST $ useFold fold''
>
> On Nov 28, 2012, at 9:52 PM, Dmitry Kulagin <dmitry.kula...@gmail.com>
> wrote:
>
> > Hi Cafe,
> >
> > I try to implement some sort of monadic fold, where traversing is
> polymorphic over monad type.
> > The problem is that the code below does not compile. It works with any
> monad except for ST.
> > I suspect that monomorphism is at work here, but it is unclear for me
> how to change the code to make it work with ST.
> >
> > fold :: Monad m => (Int -> m ()) -> m ()
> > fold f = mapM_ f [0..20]
> >
> > selectFold :: Monad m => String -> IO ((Int -> m ()) -> m ())
> > selectFold method = do
> > -- in real program I'd like to choose between
> > -- different fold methods, based on some IO context
> > return fold
> >
> > useFold :: Monad m => ((Int -> m ()) -> m ()) -> m ()
> > useFold fold' = fold' f
> > where f _ = return () -- some trivial iterator
> >
> > main = do
> > fold'' <- selectFold "some-method-id"
> > print $ runST $ useFold fold''
> >
> >
> > Thank you!
> > Dmitry
> > _______________________________________________
> > Haskell-Cafe mailing list
> > Haskell-Cafe@haskell.org
> > http://www.haskell.org/mailman/listinfo/haskell-cafe
>
>
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