Does ghc make any attempt to analyze dataflow and eliminate unnecessary copying for incremental updates? I compile the code below with -O, and based on the time it takes to run I'd guess it's copying the array with every update, even though there's no need in this particular case to do that. Perhaps there's a compiler flag or pragma to turn on the analysis?

module Main where import Array main = print (take 1 (reverse (elems (array_neg (setup 100000))))) setup i = listArray (1,i) [1..i] -- the next three lines are the important part. array_neg array = array_neg2 (length (elems array)) array array_neg2 0 array = array array_neg2 i array = array_neg2 (i-1) (array // [(i,-i)])

I realize I can force it to stop copying if I use a Monad, and there are of course more efficient ways to code this contrived testcase. I want to know if ghc can make it efficient as it stands.

GHC doesn't do this kind of optimisation, no. There is the DiffArray type which provides constant-time array update, although it has been reported to have a high constant factor overhead. Your best bet for algorithms using mutable arrays is to encapsulate it in a state thread with the ST monad. Cheers, Simon