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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