Mine

module Main where

import Text.ParserCombinators.Parsec
import Text.ParserCombinators.Parsec.Token
import Data.Char

price   :: Parser Int   -- price in cents
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

receipt :: Parser Bool
receipt = do{ ps <- many produkt
            ; p  <- total
            ; return (sum ps == p)
            }

produkt = do{ reserved "return"
            ; p <- price
            ; semi
            ; return (-p)
            }
      <|> do{ identifier
            ; p  <- price
            ; semi
            ; return p
            }
      <?> "produkt"

total   = do{ p <- price
            ; reserved "total"
            ; return p
            }
runLex :: Show a => Parser a -> String -> IO ()
runLex p input
        = run (do{ whiteSpace
                 ; x <- p
                 ; eof
                 ; return x
                 }) input


run :: Show a => Parser a -> String -> IO ()
run p input
        = case (parse p "" input) of
            Left err -> do{ putStr "parse error at "
                          ; print err
                          }
            Right x  -> print x

---
Compiling Main             ( rec.hs, interpreted )

rec.hs:8:10:
    Expecting a function type, but found `GenParser tok st a'
      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)))'

rec.hs:24:23:
    Couldn't match `TokenParser st' against `[Char]'
      Expected type: TokenParser st
      Inferred type: [Char]
    In the first argument of `reserved', namely `"return"'
    In a 'do' expression: reserved "return"

rec.hs:37:23:
    Couldn't match `TokenParser st' against `[Char]'
      Expected type: TokenParser st
      Inferred type: [Char]
    In the first argument of `reserved', namely `"total"'
    In a 'do' expression: reserved "total"

rec.hs:43:19:
    Couldn't match `GenParser Char ()' against `(->) (TokenParser st)'
      Expected type: GenParser Char ()
      Inferred type: (->) (TokenParser st)
    In a 'do' expression: whiteSpace
    In the first argument of `run', namely
        `(do
            whiteSpace
            x <- p
            eof
            return x)'
Failed, modules loaded: none.