[Haskell-cafe] parsec manyTill stack overflow

Derek Elkins derek.a.elkins at gmail.com
Fri Jul 4 17:22:06 EDT 2008

On Fri, 2008-07-04 at 13:31 -0700, Badea Daniel wrote:
> I'm trying to parse a large file looking for instructions on each line and for a section end marker but Parsec's manyTill function causes stack overflow, as you can see in the following example (I'm using ghci 6.8.3):
> > parse (many anyChar) "" ['a'|x<-[1..1024*64]]
> It almost immediately starts printing "aaaaaaaaaaa...." and runs to completion.
> > parse (manyTill anyChar eof) "" ['a'|x<-[1..1024*1024]]
> *** Exception: stack overflow
> I guess this happens because manyTill recursively accumulates output
> from the first parser and returns only when it hits the 'end' parser.
> Is it possible to write a version of 'manyTill' that works like 'many'
> returning output from 'anyChar' as soon as it advances through the
> list of tokens?

No, manyTill doesn't know whether it is going to return anything at all
until its second argument succeeds.  I can make manyTill not stack
overflow, but it will never immediately start returning results.  For
the particular case above you can use getInput and setInput to get a
result that does what you want.

parseRest = do
    rest <- getInput
    setInput []
    return rest

That should probably update the position as well though it's not so
crucial in the likely use-cases of such a function.

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