forall type gives "not polymorphic enough" error incorrectly

> type SFST s a b = a -> ST s b
> newtype SF a b = SF { runSF :: forall s. ST s (SFST s a b) }

When making this into an instance of Arrow, compose works beautifully:

> sfCompose :: SF b c -> SF c d -> SF b d
> sfCompose bc cd = SF sf where
>   sf = do
>     runBc <- runSF bc
>     runCd <- runSF cd
>     return $ \b -> runBc b >>= runCd

but pure gives an odd message:

> sfPure :: (b -> c) -> SF b c
> sfPure f = SF sf where
>     sf = return $ \b -> return (f b)

frp_typebug.lhs:20:12:
    Inferred type is less polymorphic than expected
      Quantified type variable `s' is mentioned in the environment:
	sf :: ST s (b -> ST s c) (bound at frp_typebug.lhs:21:5)
    In the first argument of `SF', namely `sf'
    In the expression: SF sf
    In the definition of `sfPure':
	sfPure f = SF sf
		 where
		     sf = return $ (\ b -> return (f b))

This can be worked around using scoped type variables:

> sfPure2 :: (b -> c) -> SF b c
> sfPure2 (f :: b -> c) = SF sf where
>     sf :: forall s. ST s (SFST s b c) 
>        = return $ \b -> return (f b)

but it's ugly.