"Could not deduce (MArray (STUArray s) Int (ST s)) from context ()" when applying runST

[Christian Klauser]

Hi, I'm in the process of learning haskell and came across this problem:

Using `Glasgow Haskell Compiler, Version 6.10.4, for Haskell 98, stage 2
booted by GHC version 6.10.1`

Common beginning of the file
============================
    {-# LANGUAGE FlexibleContexts #-}

    module UPSO where

    import Control.Monad(forM,forM_)
    import Control.Monad.ST.Lazy (ST,runST)
    import Data.Array.MArray (MArray, Ix, getBounds, newArray, readArray,
writeArray)
    import Data.Array.ST (STArray,STUArray)

    minmax xs@(x:_) = foldr (\x (l,u) -> (min x l,max x u)) (x,x) xs

    modify a i f = do
        x <- readArray a i
        writeArray a i (f x)
     
    increment a i = modify a i (+1)
    decrement a i = modify a i (\x -> x - 1)

    uniquePermutationsM t 0 = return $! [[]]
    uniquePermutationsM t pos = do
        (l,u) <- getBounds t
        perms <- forM [l..u] (\d -> do
             count <- readArray t d -- t[d]
            if count == 0
                then return $! []
                else do
                    decrement t d
                    pss <- uniquePermutationsM t (pos-1)
                    increment t d
                    return $! (map (d:) pss)
            )
        return $! (concat perms)
    
Using STArray (works)
=====================
    mkArray :: (Int,Int) -> (ST s) (STArray s Int Int)    
    mkArray bounds = newArray bounds 0 

    uniquePermutationsST ::  [Int] -> ST s [[Int]]
    uniquePermutationsST xs = do
        let bounds@(l,u) = (minmax xs) 
        t <- mkArray  bounds
        forM_ xs (increment t)
        pos <- sum `fmap` mapM (readArray t) [l..u]
        uniquePermutationsM t pos

    uniquePermutations xs = runST (uniquePermutationsST xs)

Using STUArray (doesn't work)
=============================
But when I try to switch to unboxed arrays, I get an error message.

    mkArray :: (Int,Int) -> (ST s) (STUArray s Int Int)    
    mkArray bounds = newArray bounds 0 

    uniquePermutationsST ::  [Int] -> ST s [[Int]]
    uniquePermutationsST xs = do
        let bounds@(l,u) = (minmax xs) 
        t <- mkArray  bounds
        forM_ xs (increment t)
        pos <- sum `fmap` mapM (readArray t) [l..u]
        uniquePermutationsM t pos

    uniquePermutations xs = runST (uniquePermutationsST xs)

Error messages
==============

    Could not deduce (MArray (STUArray s) Int (ST s))
      from the context ()
      arising from a use of 'newArray' at UPSO.hs:35:17-33
    Possible fix:
      add (MArray (STUArray s) Int (ST s)) to the context of
        the type signature for 'mkArray'
      or add an instance declaration for (MArray (STUArray s) Int (ST s))
    In the expression: newArray bounds 0
    In the definition of 'mkArray': mkArray bounds = newArray bounds 0

and also:

    Could not deduce (MArray (STUArray s) Int (ST s))
      from the context ()
      arising from a use of 'increment' at UPSO.hs:41:14-24

After almost two hours of fiddling with the type annotations I hope someone
can point me in the right direction. What on earth is going wrong?

Thank you for your time.