I was trying to implement quicksort using Data.Array.ST. I can't find a typing for qsortIndices in the following module: <file name="STUArraySort.hs"> {-# OPTIONS_GHC -fglasgow-exts #-} module STUArraySort where import Control.Monad.ST import Data.Array.IArray import Data.Array.MArray import Data.Array.ST import Data.Array.Unboxed mkIndices (arr) = (newListArray (bounds arr) (indices arr)) qsortIndices :: (Ord e, IArray a e, MArray (STUArray s) i (ST s)) => a i e -> UArray i i qsortIndices arr = runSTUArray $ do mkIndices arr </file> When I try to load this in GHCi, I get a nasty type error -- I even get this if I take out the type signature. I noticed a comment in Data.Array.ST that looks quite related: -- INTERESTING... this is the type we'd like to give to runSTUArray: -- -- runSTUArray :: (Ix i, IArray UArray e, -- forall s. MArray (STUArray s) e (ST s)) -- => (forall s . ST s (STUArray s i e)) -- -> UArray i e -- -- Note the quantified constraint. We dodged the problem by using -- unsafeFreezeSTUArray directly in the defn of runSTUArray above, but -- this essentially constrains us to a single unsafeFreeze for all STUArrays -- (in theory we might have a different one for certain element types). In a similar vain, I think that the signature I want would be: qsortIndices :: (Ord e, IArray a e, forall s. MArray (STUArray s) i (ST s)) => a i e -> UArray i i Unfortunately, this is not even syntactically valid... -- Thanks, Sam