Hi, Since I discovered that my program spent most of its time reading and writing files, I had a go at transforming it from using readFile to using array IO. Since my file consists of lines of length 18, I tried writing a similar function to readFile, to return a lazy list of lines as arrays, included below. This works, but wasn't more efficient. I haven't toyed to much with this kind of thing before, so please let me know if I'm doing anything obviously stupid. Perhaps the whole strategy is wrong? Should I wrap more of my program in IO, and do things incrementally there? (The files can be large, typically 100M-2G range, so reading them strictly is not an option) Or would it be more efficient to read larger chunks at a time? If so, what is a good chunk size? 1K? 4K? 1M? ----------8<-------------------- -- read the external file as a lazy list of arrays readXFile :: FilePath -> IO [UArray Int Word8] readXFile f = do h <- openFile f ReadMode getArrays h where getArrays h = do end <- hIsEOF h case end of True -> return [] False -> do (a :: IOUArray Int Word8) <- newArray (0,17) 0 hGetArray h a 18 a' <- unsafeFreeze a as <- unsafeInterleaveIO (getArrays h) return (a':as) ----------8<-------------------- -kzm -- If I haven't seen further, it is by standing in the footprints of giants