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