Re: [Haskell-cafe] Parsec Problem

27 Jul 2004

      On Mon, 26 Jul 2004 22:45:50 -0500,  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:
...
mkTP :: 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
ghci or hugs
$ 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

Re: [Haskell-cafe] Parsec Problem

Georg Martius