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

[MigMit]

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.kulagin at 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
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