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
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