[Haskell] Help in understanding a type error involving forall and class constraints

[MR K P SCHUPKE]

Erm, something I remember about needing MArrays... Here's something
which does the same thing (cooks and MArray then freezes it - also using
STUArray... The neat thing is MArray is a class, so you can swap between
STUArray and STArray implementations without changing code.

This is the classic dynamic programming version of string difference:

distString :: String -> String -> Int
distString s0 s1 = runST (difST s0 s1)

difST :: MArray (STUArray s) Int (ST s) => String -> String -> ST s Int
difST s0 s1 = do
   b@(_,br) <- return $ (\x1 y1 -> ((0,0),(x1,y1))) (length s0) (length s1)
   d <- newArray b 0 :: ST s (STUArray s (Int,Int) Int)
   mdiff d s0 s1
   readArray d br

mMin :: Int -> Int -> Int -> Int
mMin i j k = min (min i j) k

costDelete :: Char -> Int
costDelete _ = 1

costInsert :: Char -> Int
costInsert _ = 1

costReplace :: Char -> Char -> Int
costReplace _ _  = 1

mdiff :: MArray a Int m => a (Int,Int) Int -> String -> String -> m ()
mdiff (d :: a e i) s0 s1 = do
   writeArray d (0,0) 0
   foreach 1 s0 $ \x a -> do
      dx <- readArray d (x-1,0)
      writeArray d (x,0) (dx+costDelete a)
   foreach 1 s1 $ \y b -> do
      dy <- readArray d (0,y-1)
      writeArray d (0,y) (dy+costInsert b)
   foreach 1 s0 $ \x a -> do
      foreach 1 s1 $ \y b -> do
         dx <- readArray d (x-1,y)
         dy <- readArray d (x,y-1)
         dxy <- readArray d (x-1,y-1)
         writeArray d (x,y) $ mMin (dx+costDelete a) (dy+costInsert b)
            (if a==b then dxy else dxy+costReplace a b)
   where

   foreach :: MArray a Int m => Int -> String -> (Int -> Char -> m ()) -> m ()
   foreach _ [] _ = return ()
   foreach i (c0:cs) f = do
      f i c0
      foreach (i+1) cs f


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Keean.