Am Samstag, 21. Februar 2009 21:04 schrieb Colin Paul Adams:
The tutorials I've found don't tell me what I want to know :-(
I have lines that begin with an integer, then white-space, and then further structure to the end-of-line.
So for a starter, could you show me how to write a parser for the integer (returning an Int), and how to combine it with a parser for the rest of the line (take that as written - it isn't, but I'll try to do that myself given such a kick-start).
Thanks.
-- could also be Parser Int, if you don't need user-state parseInt :: GenParser Char st Int parseInt = do neg <- (char '-' >> return True) <|> (optional (char '+') >> return False) digs <- many1 digit let n | neg = negate $ read digs | otherwise = read digs return n parseLine :: GenParser Char st LineStructure parseLine = do -- spaces here, if there may be whitespace at the beginning of lines num <- parseInt spaces things <- parseRemainderOfLine return (combine num things) The strategy is always the same, you write parsers for small things and combine their results. Note that things are somewhat tricky if the structure you want to parse isn't very strict, if one of your possible parsers at some point in the parsing process can fail, but not immediately and you need to backtrack, you have to use the 'try' combinator as in combo = (try parser1) <|> parser2 , but try is bad for performance, so use it only where it's necessary. The latter part is notoriously difficult to identify for the inexperienced, so until you know parsec better, it's okay to insert a few 'try's more than necessary. HTH, otherwise ask again, Daniel