robert bauer wrote:

I have an IOArray, but it doesn't work the way I expected it to.

I defined

newMem s = newIOArray (0, size-1) 0

and then

x = newMem 30

then

do { y <- x ; v <- readIOArray y 2 ; print v ; writeIOArray y 2 20 ; v <- readIOArray y 2 ; print v }

this resulted in 0, 2 as expected.

however, then

do { y <- x ; v <- readIOArray y 20 ; print v }

results in 0

I thought the IOArray was basically a static structure?

yes it is. However, 'x' is not an IOArray. Its type is (after defaulting) x :: IO (IOArray Int Int) It represents an IO action that constructs a *new* array.> do { so

do { y <- x ; v <- readIOArray y 20 ; print v }

constrcts a new array and finds its 20th element, which was initialized to 0. The problem you have is dealing with global variables in Haskell. There are two major approaches. a) Construct the variable once and pass it explicitely as an argument to all functions. (In other words, don't use global variables at all.)

main = do { mem <- newMem func1 mem func2 mem }

This is simple, easy to understand, but it doesn't scale well. To make it scale better you can define a data type for the global state.

data GlobalState = GS { mem :: IOArray Int Int }

This record can then easily be extended with new fields as they come up. Finally you can get rid of the explicit 'mem' (or 'state') arguments by using a state monad transformer (StateT). b) Use actual global variables. See http://haskell.org/haskellwiki/Top_level_mutable_state for further information. http://haskell.org/haskellwiki/Global_variables is also interesting. HTH, Bertram