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[Haskell-cafe] Re: [darcs-users] Iteratees, streams, and mmap

Subject: [Haskell-cafe] Re: [darcs-users] Iteratees, streams, and mmap
From: Jason Dagit
Date: Sat, 12 Dec 2009 10:13:44 -0800
Hi Heinrich,

On Sat, Dec 12, 2009 at 2:42 AM, Heinrich Apfelmus <[email protected]> wrote:
Jason Dagit wrote:
> My next experiment will be to find ways to express "take this operation and
> apply it to a stream without letting the stream leak". One implication is
> that gzReadFilePS should not be used outside of a core set of modules which
> have been auideted to be resource concious.  Another implication is that we
> need to be really careful about wether or not we allow returning of
> sequences of patches.  Possibly, we need several foldl-like functions that
> open the stream internally.  For example, to process the pending maybe we
> should have:
> 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.

Just a small comment on a potential flaw in this scheme and the
observation that even the rank-2 type trick from the  ST s  monad
wouldn't help.

I would say it does help, but it doesn't make it perfect.

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.

  withPending (flip (:))

Fortunately, the type system can ensure that the patches don't leak.

  withPending :: (forall s. a -> Patch s -> a) -> IO a

Now,  a  may not mention  s  and the type checker will reject  flip (:)
 as argument. See also

 Oleg Kiselyov, Chung-chieh Shan.
 Lightweight Monadic Regions.

for an elaboration of this technique.

I'm still on the fence as to whether this style of writing it will add value greater than the complexity it brings.  I am certainly considering it :)  The darcs source does other things that are also fairly complex.

However, the line between leaking and not leaking is very thin here. As
soon as we are given for example a function

  name :: Patch s -> String

that discards the  s , its results can "leak", in the sense that we
could now build a list of names

  foo :: IO [String]
  foo = withPending . flip $ (:) . name

Even worse, any type  a  that doesn't have O(1) space usage will "leak"

  bar :: IO [()]
  bar = withPending . flip $ const (() :)

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

I'm open to suggestions on how to ensure the code has the space behavior I want.  Lazy IO* and streams of patches is more compositional and natural to Haskell programmers, but it seems that it's too hard to ensure the code has reasonable space usage.  At least where the darcs source is concerned.  Therefore, I think the status quo demonstrates that in the darcs source it's worth experimenting with alternatives to lazy io and streams.  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.

(*) Note: Lazy IO itself is used in very few places in darcs these days because it has lead to serious bugs.  These days me point is more about big streams getting retained.  Finding where and why has proven difficult.

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