Bug in writeArray?

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Grzegorz Chrupała

23 Sep 2009 23 Sep '09

11:19 a.m.

Hi all, This seems like a bug in the implementation of writeArray: when passed an out-of-range index it silently writes to an incorrect index in the array. -- import Data.Array.IO import Data.Array.Unboxed main = do let (l,u) = ((0,10),(20,20)) marr <- newArray (l,u) 0 :: IO (IOUArray (Int,Int) Int) let badi = (10,9) print (inRange (l,u) badi) writeArray marr badi 1 arr <- freeze marr :: IO (UArray (Int,Int) Int) print . filter ((/=0) . snd) . assocs $ arr -- Grzegorz

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Bulat Ziganshin

23 Sep 23 Sep

11:36 a.m.

Hello Grzegorz, Wednesday, September 23, 2009, 7:19:59 PM, you wrote:

...

This seems like a bug in the implementation of writeArray: when passed let (l,u) = ((0,10),(20,20))

writeArray computes raw index (from 0 to total number of array elements) and check that this index is correct. with multi-dimensional arrays this approach may lead to wrong results, as you mentioned. it's known problem that isn't fixed for a long time probably due to efficiency cautions. the error is here: data Ix i => Array i e = Array !i -- the lower bound, l !i -- the upper bound, u !Int -- a cache of (rangeSize (l,u)) -- used to make sure an index is -- really in range (Array# e) -- The actual elements (!) :: Ix i => Array i e -> i -> e arr@(Array l u n _) ! i = unsafeAt arr $ safeIndex (l,u) n i safeIndex :: Ix i => (i, i) -> Int -> i -> Int safeIndex (l,u) n i = let i' = unsafeIndex (l,u) i in if (0 <= i') && (i' < n) then i' else error "Error in array index" obviously, safeIndex should use inRange call instead -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

Grzegorz Chrupała

24 Sep 24 Sep

7:53 a.m.

2009/9/23 Bulat Ziganshin :

...

Hello Grzegorz,

Wednesday, September 23, 2009, 7:19:59 PM, you wrote:

...
This seems like a bug in the implementation of writeArray: when passed let (l,u) = ((0,10),(20,20))

writeArray computes raw index (from 0 to total number of array elements) and check that this index is correct. with multi-dimensional arrays this approach may lead to wrong results, as you mentioned. it's known problem that isn't fixed for a long time probably due to efficiency cautions.

Hmm, I understand that efficiency is an issue, but in that case shouldn't unsafe writing be provided by and unsafeWriteArray function, while writeArray does proper range checking? Or at least this problem with writeArray should be clearly indicated in the documentation. I for one spent several hours debugging before finding out about this lack of proper range checks so it's not an imaginary problem. -- Grzegorz

Iavor Diatchki

12:32 p.m.

I agree with Grzegorz. Perhaps we should file a bug-report, if there isn't one already? -Iavor 2009/9/24 Grzegorz Chrupała :

...

2009/9/23 Bulat Ziganshin :

...
Hello Grzegorz,

Wednesday, September 23, 2009, 7:19:59 PM, you wrote:

...
This seems like a bug in the implementation of writeArray: when passed let (l,u) = ((0,10),(20,20))

writeArray computes raw index (from 0 to total number of array elements) and check that this index is correct. with multi-dimensional arrays this approach may lead to wrong results, as you mentioned. it's known problem that isn't fixed for a long time probably due to efficiency cautions.

Hmm, I understand that efficiency is an issue, but in that case shouldn't unsafe writing be provided by and unsafeWriteArray function, while writeArray does proper range checking?

Or at least this problem with writeArray should be clearly indicated in the documentation. I for one spent several hours debugging before finding out about this lack of proper range checks so it's not an imaginary problem.

-- Grzegorz _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

David Menendez

1:09 p.m.

2009/9/24 Iavor Diatchki :

...

I agree with Grzegorz. Perhaps we should file a bug-report, if there isn't one already?

http://hackage.haskell.org/trac/ghc/ticket/2120 Apparently, it's fixed in GHC 6.12. -- Dave Menendez http://www.eyrie.org/~zednenem/

Duncan Coutts

12:51 p.m.

On Thu, 2009-09-24 at 13:53 +0200, Grzegorz Chrupała wrote:

...

2009/9/23 Bulat Ziganshin :

...
Hello Grzegorz,

Wednesday, September 23, 2009, 7:19:59 PM, you wrote:

...
This seems like a bug in the implementation of writeArray: when passed let (l,u) = ((0,10),(20,20))

writeArray computes raw index (from 0 to total number of array elements) and check that this index is correct. with multi-dimensional arrays this approach may lead to wrong results, as you mentioned. it's known problem that isn't fixed for a long time probably due to efficiency cautions.

Hmm, I understand that efficiency is an issue, but in that case shouldn't unsafe writing be provided by and unsafeWriteArray function, while writeArray does proper range checking?

Or at least this problem with writeArray should be clearly indicated in the documentation. I for one spent several hours debugging before finding out about this lack of proper range checks so it's not an imaginary problem.

It's now fixed: http://hackage.haskell.org/trac/ghc/ticket/2120#comment:13 Duncan

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Bulat Ziganshin
David Menendez
Duncan Coutts
Grzegorz Chrupała
Iavor Diatchki