[Haskell] How to use STArray?

[Benjamin Franksen]

Hmmm, no answer on cafe, maybe someone here with a good idea?

----------  Forwarded Message  ----------

On Thursday 25 August 2005 19:58, Udo Stenzel wrote:
> [...] you'll need a type signature somewhere to help ghc resolve
> the overloading of newArray and readArray, which is surprisingly
> tricky due to the "s" that must not escape.  This works:
>
> compute :: Int -> Int
> compute n = runST ( do
>     arr <- newArray (-1, 1) n :: ST s (STArray s Int Int)
>     readArray arr 1
>   )

Hello,

I am fighting with a similar problem. I want to use STUArray but
without committing to a fixed element type. For instance (this is not my 
real problem, but it's similar and easier to motivate), here is a
function that appends two UArrays:

A little helper first

> copy :: (MArray a e m, IArray b e) =>
>         a Int e -> Int -> b Int e -> Int -> Int -> m ()
> copy dest destix src srcix cnt
>
>   | cnt <= 0  = return ()
>   | otherwise = do
>
>       writeArray dest destix (src ! srcix)
>       copy dest (destix+1) src (srcix+1) (cnt-1)

and here is the append function

> append :: UArray Int e -> UArray Int e -> Int -> UArray Int e
> append x y low = runSTUArray (do
>     z <- newArray_ (low,low+len x+len y)
>     copy z low x (first x) (len x)
>     copy z (low+len x) y (first y) (len y)
>     return z)
>   where
>     len = rangeSize . bounds
>     first = fst . bounds

Of course this can't work, because 'copy' needs the MArray and IArray
contexts:

    No instance for (MArray (STUArray s) e (ST s))
      arising from use of `copy' at Problem.lhs:31:7-10
      [...]
    No instance for (IArray UArray e)
      arising from use of `copy' at Problem.lhs:31:7-10
      [...]

But now, when I add

> append :: (IArray UArray e, MArray (STUArray s) e (ST s)) => ...

I still get the same error message regarding the MArray constraint:

    No instance for (MArray (STUArray s) e (ST s))
      arising from use of `copy' at Problem.lhs:31:7-10

What am I missing? That is, how and where do I have to specify the 
constraint?

Ben