Not only does your suggestion make more sense than what I was doing, but also it causes the 'matrices' to behave as expected, namely to have the side effects incorporated into their array values. I can't say I fully understand why this is true. In both cases I was wrapping Rmatrix in a monad after every computation. The difference is that the array component had an additional monad wrapper around and now it doesn't. That is, old case newtype Rmatrix = Rmatrix (Int, Int, IO(StorableArray Int CDouble)) new case newtype Rmatrix = Rmatrix (Int, Int, StorableArray Int CDouble) In the old way, the additional IO wrapper forces an additional nesting of do loops for most processing. It is certainly the case that the do loop that performs the withStorableArray operations has the contents of the array modified. Outside that loop, once all actions on the monad are done, I am not convinced the array will mutate unless it's explicitly returned. Now hopefully I won't induce unnecessary copying. Thanks again for your help. Brian Hulley wrote:

Hi Matthew - Sorry about the long delay in my reply to your last message - I'd composed the reply but when I tried to send it I was blocked by spamhaus.com because the dynamic IP that my ISP had allocated to me at the time I was connected had been listed on some composite spam blocker database, and even when I tried reconnecting several times I kept ending up being allocated the same client IP. So I did a complete scan of my system and now (several hours later) when I tried to reconnect again, I've been allocated a different client IP so my reply to your last message was sent at last! ...

I hope it answers your question relating to the above: the array is not created in the let statement, but twice, in rout <- mac r2 (-0.5) rmat and then again in mprint r2

Rmatrix really needs to have type (Int, Int, StorableArray Int CDouble) if your intention is to pass around an actual allocated matrix together with its dimensions, and listtoRmatrix would need to be typed as:

listtoRmatrix :: Int -> Int -> IO Rmatrix

so that the let statement could be replaced by

r2 <- listtoRmatrix 3 2 [1, 1, 1, 1, 1, 1]

and for example mprint would be defined as:

mprint :: Rmatrix -> IO () mprint (Rmatrix cols rows arr) = do -- just print element at (0,0) for the moment readArray arr 0 >>= print

(Of course the above needs to iterate through the rows cols etc but you've already got the code to do this...)

Hope this helps,

Brian.