On Sun, Jul 5, 2009 at 7:46 PM, Uwe Hollerbach<uhollerb...@gmail.com> wrote: > On 7/5/09, Paul L <nine...@gmail.com> wrote: >> Previously you had lastOrNil taking m [a] as input, presumably >> generated by mapM. So mapM is actually building an entire list before >> it returns the argument for you to call lastOrNil. This is where you >> had unexpected memory behavior. >> >> Now you are "fusing" lastOrNil and mapM together, and instead of >> building a list, you traverse it and perform monadic action along the >> way. This can happen in a constant memory if the original pure list is >> generated lazily. >> >> I think the real problem you had was a mis-understanding of mapM, and >> there was nothing wrong with your previous lastOrNil function. mapM >> will only return a list after all monadic actions are performed, and >> in doing so, it inevitably has to build the entire list along the way. >> >> -- >> Regards, >> Paul Liu >> >> Yale Haskell Group >> http://www.haskell.org/yale > > Hi, Paul, thanks for the comments. You're quite right that I am fusing > the two functions together, but I think I wasn't mis-understanding > mapM... I knew I was generating the entire list, and aside from the > slight inefficiency of generating it only to tear it down an instant > later, that would have been no problem. But I was expecting all of the > memory associated with the list to be reclaimed after I had processed > it, and that was what was not happening as far as I could tell. (This > isn't one monolithic list, by the way; it's the small bodies of a > couple of small scheme functions that get evaluated over and over. So > the setup and teardown happens a lot.) I don't have very good > intuition yet about what should get garbage-collected and what should > get kept in such situations, and in fact I'm kind of in the same boat > again: the test case now runs much better, but it still leaks memory, > and I am again stumped as to why. Could I see something useful by > examining ghc core? I haven't looked at that yet, no idea what to look > for... > > Uwe > _______________________________________________
mapM_ might be useful to you. I know there are cases where mapM leaks memory but mapM_ doesn't, basically because mapM_ throws away all of the intermediate results immediately. You might want to condition on nullness of the list and then mapM_ your function over the init of the list and then just return the function on the last element of the list. Alex _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
