On Mon, 26 Jul 2004 22:45:50 -0500, <[EMAIL PROTECTED]> wrote:
Hello,
I copied this example exactly from the page
http://www.cs.uu.nl/people/daan/download/parsec/parsec.html
-----begin----- module Parser where import Data.Char import Text.ParserCombinators.Parsec import Text.ParserCombinators.Parsec.Char import Text.ParserCombinators.Parsec.Token price = lexeme (do{ ds1 <- many1 digit ; char '.' ; ds2 <- count 2 digit ; return (convert 0 (ds1 ++ ds2)) }) <?> "price" where convert n [] = n convert n (d:ds) = convert (10*n + digitToInt d) ds -----end-----
However attempting to compile it gives the error message
Test.hs:8: Couldn't match `GenParser tok st a' against `CharParser st1 a1 -> CharParser st1 a1' Expected type: GenParser tok st a Inferred type: CharParser st1 a1 -> CharParser st1 a1 Probable cause: `lexeme' is applied to too few arguments in the call (lexeme (do ds1 <- many1 digit char '.' ds2 <- count 2 digit return (convert 0 (ds1 ++ ds2)))) In the first argument of `(<?>)', namely `lexeme (do ds1 <- many1 digit char '.' ds2 <- count 2 digit return (convert 0 (ds1 ++ ds2)))'
lexeme is now a record accessor of TokenParser. The idea is that you define a language with a specific comment style, reserved names, operators and so on and then let Parsec do the hard work for you :-). Basicly: 1) use makeTokenParser with a language definition to define a TokenParser 2) give "price" the TokenParser as first argument and pass it to lexeme or whitespace or whatever there is in Text.ParserCombinators.Parsec.Token
Here a version that uses TokenParser:
ghci or hugsmkTP :: TokenParser st mkTP = makeTokenParser $ emptyDef { commentStart = "{-" , commentEnd = "-}" , commentLine = "--" , nestedComments = True , identStart = lower <|> char '_' ... }
price :: TokenParser () -> Parser Double price tp = do whiteSpace tp val <- float tp return val
$ parse (price mkTP) "" "1.23" Right 1.23 $ parse (price mkTP) "" " {-comment-} 1.23" Right 1.23
Okay my version of price doesn't check if there are exactly 2 digits after the point but it can handle comments ;-).
Hope it helped!
Georg _______________________________________________ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe