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

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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 (() :)

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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

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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.

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

Jason Dagit wrote:

Heinrich Apfelmus wrote:

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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.

On second (and late) thought, this idea is not as good as it sounds and needs more thought. The thing is if A is an instance of Small , we are not guaranteed that a value x :: A will take little memory, we are only guaranteed that its *normal form* takes little memory. Now, due to pervasive lazy evaluation, it's quite impossible to track whether a value is in normal form in the type system.

It seems like we also need: instance Small (IO ())

Which is not necessarily bounded due to things like IORefs. [...]

In the light of the above, I think this is related to the issue that we can't evaluate IO () to normal form at all, i.e. there is no deepSeq for values of type IO () .

I could also see us needing this:

bracketSmall :: (Small c) => IO a -> (a -> IO b) -> (a -> IO c) -> IO c

I'm not sure if b needs to be Small, since it's just supposed to be the return value of the deallocation step.

Thanks to parametricity, we know that b is thrown away, it can never appear in the result type c .

It seems that having functions like bracketSmall are necessary if we want to hide the stream itself from being exposed outside of the traversal function. For example, your foldlSmall doesn't leak, but something at the same level of scope as it could leak the list.

Yep, though this does have the unfortunate (?) consequence that we cannot use bracketSmall to return any large values anymore, even if they do not leak the stream. (A notion which literally gets "muddier" in the course of the program because it might be that c does not leak the vanilla stream, but for example every character of it.) Regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com