Iteratee and parsec (was: Re: [Haskell-cafe] Re: Haskell-Cafe Digest, Vol 78, Issue 14)

On Thu, 2010-02-11 at 13:34 -0600, Antoine Latter wrote:

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On Thu, 2010-02-11 at 11:00 -0500, Gregory Collins wrote:

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Maciej Piechotka writes:

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On Tue, 2010-02-09 at 16:41 +0000, John Lato wrote:

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See http://inmachina.net/~jwlato/haskell/ParsecIteratee.hs for a

valid

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Stream instance using iteratee. Also Gregory Collins recently

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an iteratee wrapper for Attoparsec to haskell-cafe. To my knowledge these are not yet in any packages, but hackage is vast.

Hmm. Am I correct that his implementation caches everything?

The one that John posted (iteratees on top of parsec) has to keep a copy of the entire input, because parsec wants to be able to do arbitrary backtracking on the stream.

Well. Not quite. AFAIU (and ByteString implementation indicate so)

On Thu, Feb 11, 2010 at 1:27 PM, Maciej Piechotka wrote: posted the

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uncons have a type uncons :: s -> m (Maybe (t, s))

Where s indicates the position on the stream. Since it is impossible to get back from having s alone the GC should be free to finalize all memory allocated to cache the stream before the first living s.

I'm not sure that this is correct - parsec believes that it is free to call 'uncons' multiple times on the same value and receive an equivalent answer.

That's what I meant. But it have to keep the reference to the first element. Consider example with list: text = 'L':'o':'r':'e':'m':' ':'i':'p':'s':'u':'m':[] ^ ^ ^ s1 s2 s3 uncons s1 == Identity (Just ('e', 'm':' ':'i':'p':'s':'u':'m':[])) uncons s2 == Identity (Just ('p', 's':'u':'m':[])) uncons s3 == Identity Nothing However we will never get (nor we keep reference to): 'L':'o':'r':'e':'m':' ':'i':'p':'s':'u':'m':[], 'o':'r':'e':'m':' ':'i':'p':'s':'u':'m':[] 'r':'e':'m':' ':'i':'p':'s':'u':'m':[] so those values can be freed as they are before first pointer (namely s1). Regards