Consider the following GHCi session: GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> :m Data.Array.IO Prelude Data.Array.IO> t <- newArray ((0,0),(5,4)) 0 :: IO (IOUArray (Int,Int) Int) Loading package array-0.1.0.0 ... linking ... done. Prelude Data.Array.IO> getBounds t ((0,0),(5,4)) Prelude Data.Array.IO> writeArray t (10,10) 5 *** Exception: Error in array index Prelude Data.Array.IO> writeArray t (7,3) 5 *** Exception: Error in array index Prelude Data.Array.IO> writeArray t (3,7) 5 Prelude Data.Array.IO> So the array has 0 <= x <= 5 and 0 <= y <= 4, and writing to (10,10) or (7,3) throws an exception. However, writing to (3,7) triggers no exception - despite being clearly out of range. Judging by the behaviour of the large, complex program I was debugging when I stumbled upon this, the coordinates "wrap round" to the next column. (!!) Obviously, writing to non-existent coordinates and not getting an exception is a Very Bad Thing. I was counting on writeArray (and readArray) to detect out-of-range coordinates so I could fix my code. But as you can see, it doesn't actually work as advertised. You have *no idea* how long I spent trying to track this bug down! >_< Is this a known bug? Is it likely to be fixed any time soon? (I'm guessing the bug is as simple is converting indicies to integers and then checking the integers are in-range, rather than the underlying index type.)