On Tuesday 30 August 2005 06:32, oleg@pobox.com wrote:
Benjamin Franksen wrote:
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 )
I am fighting with a similar problem. I want to use STUArray but without committing to a fixed element type.
That problem has been addressed in a message http://www.haskell.org/pipermail/haskell-cafe/2004-July/006400.html which discussed several solutions. Given below is one of the solutions adjusted to fit the question of the original poster. His code is almost unchanged.
Gosh, it took me a while before I really understood /why/ your solution works, but now I think I got it. The central idea is to use an intermediate data type that has the proper constraint on its element(s). Existential quantification is not strictly necessary: if we wrap runSTUArray instead of newArray_ we merely need a rank-2 type. This also strikes me as the more direct aproach and has the additional advantage that we don't have to use unsafeFreeze. Below is the code of the modified solution. Note that there are no type signatures in the instances for class UArrayElement. \begin{code} {-# OPTIONS -fglasgow-exts #-} import Data.Array.Unboxed import Data.Array.ST import Control.Monad.ST 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) append :: (IArray a e, UArrayElement e) => a Int e -> a Int e -> Int -> UArray Int e append x y low = case freezer of Freezer f -> f (do z <- newArray_ (low,low+len x+len y-1) 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 data Freezer i e = Freezer ((forall s. MArray (STUArray s) e (ST s) => ST s (STUArray s i e)) -> UArray i e) class UArrayElement e where freezer :: Ix i => Freezer i e instance UArrayElement Bool where freezer = Freezer runSTUArray instance UArrayElement Char where freezer = Freezer runSTUArray -- ... \end{code} Ben