| > for :: Int -> IO () -> IO () | > for 0 _ = return () | > for n x = x >> for (n - 1) x Good example (allocates lots of silly thunks). I'd come across this before, and fixed the HEAD, but the 6.2 branch is still doing badly. We'll try to fix that. | Playing with the code generated by ghc is a great way to waste time | for me. Wait until you have found the RULES-pragma :-) In general, GHC's optimiser should behave predictably; small changes in the source program should not have big effects. Of course, sometimes they do, and for good reason. But not always; I'm keen to identify cases where the optimiser does a poor/unpredictable job, and fix them. So please let me know when that happens. The more you can boil down a program, and identify the culprit, as Carsten did so accurately in this case, the more likely I am to fix it. At the other end of the spectrum "this 1000 line program goes slower than I expect" is less helpful :-) Incidentally, GHC does use pre-allocated True and False booleans. Simon