Daniel Fischer wrote:
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).
Thanks Daniel! I missed out on 'guard' and somewhy was under impression that 'notFollowedBy' can only deal with Chars.
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)
Yep, that works but still provides incorrect diagnostics when fed with an empty string: Prelude Main Text.ParserCombinators.Parsec> parseTest (many1Till letter (char '.')) "" parse error at (line 1, column 1): unexpected end of input expecting "." or letter It's not a showstopper, but I'd still like to understand how to make it provide better error messages. Thanks! -- Vlad Skvortsov, vss@73rus.com, http://vss.73rus.com