[Haskell-cafe] REALLY simple STRef examples

Bulat Ziganshin bulat.ziganshin at gmail.com
Thu Jul 20 15:02:47 EDT 2006

Hello Chad,

Thursday, July 20, 2006, 9:38:43 PM, you wrote:

> I suppose the same holds for runSTUArray, right? But this still gives
> me that same error, about being less polymorphic than expected.

there is well-known problem with that _unboxed_ arrays aren't
polymorphic. Oleg Kiselyov proposed solution to that problem that i implemented
in ArrayRef library (read http://www.haskell.org/pipermail/haskell-cafe/2004-July/006400.html
and http://haskell.org/haskellwiki/Library/ArrayRef)

this code compiles (i've also fixed pair of other problems):

import Data.ArrayBZ.IArray
import Data.ArrayBZ.ST

(+=) x y = let updateX i = do xi <- readArray x i
                              writeArray x i (xi + y!i)
           in  sequence_ . map updateX $ indices x

sumArrays [] = error "Can't apply sumArrays to an empty list"
sumArrays (x:xs) = runSTUArray (result x)
    result x = do x0 <- thaw x
                  mapM_ (x0 +=) xs
                  return x0

ps: you successfully going through all the standard Haskell troubles in
this area :)  seems that making FAQ about using ST monad will be a
good idea :)

Best regards,
 Bulat                            mailto:Bulat.Ziganshin at gmail.com

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