On Wednesday 29 September 2010 19:10:22, Ben Franksen wrote:
> >
> > Note the last line mentions only '}'. I would rather like to see
> >
> > expecting "}" or digit
> >
> > since the parser could very well accept another digit here.
parsec2 did that, I don't know whether that change is intentional or
accidental.
> >
> > (1) What is the reason for this behaviour?
> > (2) Is there another combinator that behaves as I would like?
> > (3) Otherwise, how do I write one myself?
>
> I just saw that Christian Maeder answered a similar question recently. I
>
> tried his suggestion of using manyTill and bingo:
> > {-# LANGUAGE NoMonomorphismRestriction #-}
> > import Control.Applicative ((*>),(<*))
> > import Text.Parsec
> > block p = char '{' *> p <* char '}'
> > parser = block (manyTill digit (char '}'))
> > main = parseTest parser "{123a}"
>
> gives
>
> parse error at (line 1, column 5):
> unexpected "a"
> expecting "}" or digit
>
> So far so good. I wonder whether this parser is as efficient as the
> original one.
manyTill p end = scan
where
scan = do{ end; return [] }
<|>
do{ x <- p; xs <- scan; return (x:xs) }
I'm not sure, but I suspect it's less efficient.
Perhaps
manyTill' p end = scan []
where
scan acc = do { end; return (reverse acc) }
<|> do { x <- p; scan (x:acc) }
is more efficient (depends on Parsec's bind which is more efficient), you
could test.
> Also, this style is less modular, as I have to mention the
> terminator in two places.
That's not the main problem. `manyTill' consumes the ending token, so
block (manyTill whatever (char '}')) needs two '}' to succeed.
You would need
block (manyTill digit (lookAhead (char '}'))
to replicate the behaviour of block (many digit).
> Is there a non-greedy variant of 'many' so
> that modularity gets restored and efficiency is not lost?
>
> Cheers
> Ben
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