Ben Franksen wrote:
>> import Control.Applicative ((*>),(<*))
>> import Text.Parsec
>> import Text.Parsec.Char
>> block p = char '{' *> p <* char '}'
>> parser = block (many digit)
>> main = parseTest parser "{123a}"
>
> gives the output
>
> parse error at (line 1, column 5):
> unexpected "a"
> expecting "}"
>
> 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.
>
> (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. Also, this style is less modular, as I have to mention the terminator
in two places. Is there a non-greedy variant of 'many' so that modularity
gets restored and efficiency is not lost?
Cheers
Ben
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