Am Samstag 12 Dezember 2009 01:58:11 schrieb Vlad Skvortsov:
Hi,
I'm writing a parser where I find myself in need to use manyTill combinator with an additional constraint that there should be at least one meaningful element before the trailer (say, a word ended with a period: 'abc.').
Here is what I have:
many1Till :: Parser a -> Parser end -> Parser [a] many1Till p end = do notFollowedBy end p1 <- p ps <- manyTill p end return (p1:ps)
You want a nonempty sequence of 'p's which aren't 'end's, followed by an 'end'. So you could do for example a) many1Till p end = do ps <- manyTill p end guard (not $ null ps) return ps , i.e. check whether the result of manyTill is a nonempty list after the fact, or check whether manyTill p end will return an empty list before manyTill is run, like b) many1Till p end = do notFollowedBy end manyTill p end (i.e. many1Till p end = notFollowedBy end >> manyTill p end).
The problem here is that I want to catch and report a case when 'p1' could be actually parsed by 'end' ('..' is not a word ended by a period). Generally 'p' and 'end' can parse the same subset of strings.
Another version a had was:
many1Till :: Parser a -> Parser end -> Parser [a] many1Till p end = do try (end >> (unexpected "sequence terminator")) <|> (do { p1 <- p; ps <- manyTill p end; return (p1:ps) })
Here there are two disadvantages:
1) I don't like hardcoding "sequence terminator" here; 2) the error output should say that only 'p' parser is expected, while it says (technically correct) that either 'p' or 'end' is expected:
Prelude Main Text.ParserCombinators.Parsec> parseTest (many1Till letter (char '.')) "1" parse error at (line 1, column 1): unexpected "1" expecting "." or letter
(What I want here is to say "expecting letter")
For that, you need the slightly clumsier c) many1Till p end = do notFollowedBy end p1 <- p ps <- manyTill p end return (p1:ps)
Any suggestions?
Thanks!