Re: [Haskell-cafe] Re: Haskell-Cafe Digest, Vol 78, Issue 14

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

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 Stream instance using iteratee.  Also Gregory Collins recently posted 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) the 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. Maybe I'm misunderstanding what we're talking about, but a simple test is: backtrackTest = (try (string "aardvark")) <|> (string "aaple") And then attempt to parse the stream equivalent to "aaple" with 'backtrackTest'. This should be a successful parse (untested). Antoine