Re: [Haskell-cafe] Ackermann Function Memoization, GHC Weird Output or Bug?

Hello, It seems this bug has already been submitted: http://hackage.haskell.org/trac/ghc/ticket/2120 Thanks for the help. __ Donnie Jones On 3/14/08, Cale Gibbard wrote:

Here's the bug:

{-# INLINE safeIndex #-} 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"

unsafeIndex here is just a function which transforms indices into Int indices into the flat array and does no checking of validity. Then safeIndex simply checks if the result is nonnegative and less than the size of the array. Whoops! The actual test to see if the index was valid in the first place didn't actually get performed!

- Cale

On 14/03/2008, Eric Mertens wrote:

...

Smaller example of this behavior:

...

array ((0,0),(1,1)) [((1,1),6)] ! (0,3) 6

--

Eric Mertens

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