[Haskell-cafe] runST readSTRef type error

[Edward Z. Yang]

Hello Ken,

Strictly speaking, you only need Rank-2 types.  This indeed the right
way to fix the problem. 

Cheers,
Edward

Excerpts from Ken Takusagawa II's message of Wed May 04 02:00:49 -0400 2011:
> I run into the following type error:
> 
> foo :: ST s (STRef s Int) -> Int
> foo p = (runST (p >>= readSTRef))
> 
> with ghc 6.12.1
> st.hs:8:16:
>     Couldn't match expected type `s1' against inferred type `s'
>       `s1' is a rigid type variable bound by
>            the polymorphic type `forall s1. ST s1 a' at st.hs:8:9
>       `s' is a rigid type variable bound by
>           the type signature for `foo' at st.hs:7:10
>       Expected type: ST s1 (STRef s Int)
>       Inferred type: ST s (STRef s Int)
>     In the first argument of `(>>=)', namely `p'
>     In the first argument of `runST', namely `(p >>= readSTRef)'
> 
> However, if I add
> {-# LANGUAGE RankNTypes #-}
> 
> and change the type signature to
> foo :: (forall s.ST s (STRef s Int)) -> Int
> 
> it works.  I don't fully understand what's going on here.
> 
> Is this the "right" way to fix the problem?  Are there other options?
> My gut feeling is, for such a simple use case of the ST monad, I
> shouldn't need such a big hammer as RankNTypes.
> 
> --ken
>