Just did some benchmarking of FPS with ghc 6.6 from last night. Most functions seem to have gotten faster (due to the improved ForeignPtr). In particular: words 1.583 2.115 lines 0.028 0.421 inits 0.777 5.350 tails 0.836 6.634 All functions that generate many substrings. Interestingly, unpack seems to have gone backwards. unpack 3.319 1.596 So I'll have to look at that. Possibly the fusion tricks are not working? Happily this now means: main = print . length . P.lines =<< P.hGetContents stdin now runs at exactly 2x the speed of wc -l :) $ time ./a.out < 20M 314080 ./a.out < 20M 0.03s user 0.04s system 85% cpu 0.083 total pill19$ time wc -l < 20M 314080 wc -l < 20M 0.02s user 0.01s system 83% cpu 0.042 total It was around 4-5x under 6.4.1. Good work Simon! -- Don, happily benchmarking