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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
