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

12 Dec 2009

      On Sat, Dec 12, 2009 at 1:37 PM, Johann Höchtl wrote:
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Hi Heinrich,
On Sat, Dec 12, 2009 at 2:42 AM, Heinrich Apfelmus <
apfel...@quantentunnel.de> wrote:
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Jason Dagit wrote:
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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
...
...
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need to be really careful about wether or not we allow returning of
sequences of patches.  Possibly, we need several foldl-like functions
we
that
...
open the stream internally.  For example, to process the pending
maybe we
should have:
withPending :: (a -> Patch -> a) ->  IO a
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...
And withPending would start the streaming and make sure that the
stream
cannot be visible as a data dependency outside of withPending.
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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.
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Namely,  withPending  does not guarantee that the stream does not leak,
it only makes it more natural/convenient to formulate one's code so
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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.
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withPending (flip (:))
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Fortunately, the type system can ensure that the patches don't leak.
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withPending :: (forall s. a -> Patch s -> a) -> IO a
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Now,  a  may not mention  s  and the type checker will reject  flip (:)
 as argument. See also
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Oleg Kiselyov, Chung-chieh Shan.
 Lightweight Monadic Regions.
 http://www.cs.rutgers.edu/~ccshan/capability/region-io.pdfhttp://www.cs.rutgers.edu/%7Eccshan/capability/region-io.pdf
http://www.cs.rutgers.edu/%7Eccshan/capability/region-io.pdf
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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.
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However, the line between leaking and not leaking is very thin here. As
soon as we are given for example a function
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name :: Patch s -> String
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that discards the  s , its results can "leak", in the sense that we
could now build a list of names
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foo :: IO [String]
  foo = withPending . flip $ (:) . name
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Even worse, any type  a  that doesn't have O(1) space usage will "leak"
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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
On Dec 12, 7:13 pm, Jason Dagit  wrote:
that
that
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.
As a beginner to Haskell, I am only 1/3 through RWH, those lines scare
me in a sense to question my effort. I simply can not distinguish if
this discussion is somewhat pathological in a sense that every access
to the outside world imposes dangers and an additional exception
handler here and there and an additional if-statement to handle error
return codes will suffice.
We're not talking about exception handling :)  And yes, Heinrich is talking
about pathological cases.
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Or lazy evaluation, IO monads and the whole story behind
unsafePerformIO was an additional layer of self-deception and
unpredictable effects from the outside world and lazy evaluation can
NEVER be satisfactory handled.
We're not talking about unsafePerformIO either.

The discussion at hand is much simpler.  Given a large stream of data, how
should you style your code so that it's easy to reason about the space
usage?  This is an important question in every programming language I've
ever used.  Also, IO need not enter the discussion except that in darcs the
streams of data come from the disk.  I think moving the discussion to
haskell-cafe was a mistake.  I said what I said in a very specific context
(that of the darcs developers mailing list), which is likely no longer
clear.

Sorry for any distress this has caused you.

Jason