It seems the problem is indeed in rmat:

rmat n = listArray ((1,1),(n,n)) $ map ct (randoms (mkStdGen 1) ::[Bool]) where ct True = Unknown ct False = Dead

To be more precise, the problem is in randoms. If you replace rmat with the following rmat n = array ((1,1),(n,n)) $ zipWith (\ind v -> v `seq` (ind,v)) [(i,j) | i<-range, j<-range] $ map ct (randoms (mkStdGen 1) ::[Bool]) where ct True = Unknown ct False = Dead range = [1..n] then

main = putStrLn $ show $ forpaintbdry $ rmat 400

computes without problems (compiled with GHC 5.04.1, no flags). BTW, GHCi didn't have any problem even with the original code.