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