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, 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? Ben