[Haskell-cafe] runST readSTRef type error

[Ken Takusagawa II]

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