[Haskell-cafe] Stacking StateTs

[Daniel Fischer]

Am Sonntag, 22. Februar 2009 18:53 schrieb David Menendez:
> On Sun, Feb 22, 2009 at 9:20 AM, Luis O'Shea <loshea at gmail.com> wrote:
> >> test3 :: MonadState Integer m => String -> m String
> >
> > Good point.  It's interesting that this allows the signature of test5b to
> > become MonadState Integer m => m Integer (instead of (Monad m) => StateT
> > Integer (StateT String m) Integer) which is more general, and
> > (surprisingly to me) does not mention String.
>
> Odd. If I break up test5b like so:
>
> test5b = flip execStateT 0 . flip evalStateT "" $ test5bImpl
>
> test5bImpl = do
> 	modifyM test3
> 	lift . modify $ \x -> x*2 + 1
> 	modifyM test3
> 	lift . modify $ \x -> x*x
> 	modifyM test3
>
> and ask GHCi for the types, I get:
>
> *Main> :t test5bImpl
> test5bImpl :: (MonadState Integer m) => StateT String m ()
> *Main> :t test5b
> test5b :: (Monad m) => m Integer

Okay, you've found a bug in the type checker, 6.4.2 infers the types

test5b :: (Monad (StateT [Char] (StateT s m)),
           MonadState s (StateT s m),
           Num s,
           Monad m) =>
          m s
test5bImpl :: (Monad (StateT [Char] m), MonadState s m, Num s) =>
              StateT [Char] m ()

as does 6.6.1.
That does look more reasonable.