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