Then we get the problem when we use difST in distString.

I no such problem. The Glorious Glasgow Haskell Compilation System, version 6.3 ghc -H32m -Wall -O2 -fvia-C -optc-O2 -optc-march=pentium3 -optc-mfpmath=sse -fexcess-precision -fliberate-case-threshold100 -funbox-strict-fields -threaded -c Diff.hs -i../.. -syslib net +RTS -K100000000 Diff.hs: Warning: Module `Data.Array.MArray' is imported, but nothing from it is used (except perhaps instances visible in `Data.Array.MArray') Diff.hs:159:13: Warning: Defined but not used: br ...??... Of course the obvious way to have somewhere to put the class is to turn the let into a function: for example like: distString :: String -> String -> Int distString s0 s1 = f (runST (difST s0 s1)) f :: IArray a Int => a (Int,Int) Int -> Int f a = a!(1,1) ... or a non polymorphic version: f :: UArray (Int,Int) Int -> Int f a = a!(1,1) Keean.