Jason Dagit wrote:
withPending :: (a -> Patch -> a) -> IO a
And withPending would start the streaming and make sure that the stream cannot be visible as a data dependency outside of withPending.
[...]
Heinrich Apfelmus wrote:
In other words, exporting only a foldl' -like interface does not really prevent us from writing functions that have O(n) instead of O(1) space usage. But trying to rectify that with the forall s trick is a doomed idea, too.
I realize it's not perfect, but the problem we have now is that it's too easy to write things that have dismal space usage. If we can't force proper space usage, how can we make it more natural to have bounded space? Or at least a good approximation.
It seems that: * foldl'-style helps * rank-n can help * no approach I've seen *forces* the behavior we want * existing code and bug reports demonstrate we need to improve the situation
Yep, I agree completely; except for the rank-n (rank-2) trick where I'm more pessimistic. I think rank-2 works fine for things like ensuring that file handles are closed properly. But for the purpose of ensuring O(1) space usage, I think that withPending . flip $ const (() :) demonstrates that rank-2 is not worth the complexity they bring. Still, the idea of tracking resource usage in the type system is attractive.
I'm open to suggestions on how to ensure the code has the space behavior I want.
I've got another, unfinished idea:
Namely, withPending does not guarantee that the stream does not leak, it only makes it more natural/convenient to formulate one's code so that it doesn't leak. In particular, using (:) as argument pretty much defeats the whole purpose:
Right. And the iteratee library points out that your iteratees have to be well-behaved (I think there they say "bounded"). I'm well aware of this issue and thanks for pointing it out for others who are reading along.
How about tracking the requirement of "bounded" in the type system? In particular, I'm thinking of a type class class NFData a => Small a where the idea is that all types that can be stored in constant space are members of this class. For example, we have instance Small () instance Small Int instance Small Char instance (Small a, Small b) => Small (a,b) instance (Small a, Small b) => Small (Either a b) but recursive types like [a] or String are not allowed to be members. Then, withPending :: Small a => (a -> Patch -> a) -> IO a which is based on the function foldlSmall :: Small b => (b -> a -> b) -> b -> [a] -> b foldlSmall f b [] = foldlSmall f b (x:xs) = foldlSmall f b' xs where b' = rnf (f b x) is pretty much guaranteed to run in O(1) space. Well, that depends on f , but the point is it's not foldlSmall who introduces the space leak, it's the argument that takes all the blame.
In other words, the human effort to make the code behave how we want is currently too high and that's the issue I want to address. I don't know how we could make it impossible to have space leaks, although that would be interesting.
I opine that aiming for the latter (making space leaks impossible) while experimenting brings you closer to the actual goal (making them easy to avoid) when it comes to putting these experiments into practice. But that's just me. Regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com