[Haskell-cafe] Parsec Problem

[Georg Martius]

On Mon, 26 Jul 2004 22:45:50 -0500, <haskell at alias.spaceandtime.org> 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