[Haskell-cafe] following up on space leak

[Alexander Dunlap]

On Sun, Jul 5, 2009 at 7:46 PM, Uwe Hollerbach<uhollerbach at gmail.com> wrote:
> On 7/5/09, Paul L <ninegua at 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