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

[Antoine Latter]

On Thu, Feb 11, 2010 at 1:27 PM, Maciej Piechotka <uzytkownik2 at gmail.com> wrote:
> On Thu, 2010-02-11 at 11:00 -0500, Gregory Collins wrote:
>> Maciej Piechotka <uzytkownik2 at gmail.com> writes:
>>
>> > On Tue, 2010-02-09 at 16:41 +0000, John Lato wrote:
>> >>
>> >> 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