Re: [Haskell-cafe] a question about import and parsec...

21 Apr 2005

      Am Donnerstag, 21. April 2005 03:58 schrieb Greg Wolff:
...
I'm new at using Haskell and I'm trying to make use of the parsec
library.  I've started by working through the examples in the user guide
which don't work as written in ghci when I run them.  I've made
modifications that have gotten them working, up to a point.  But now I
have an error one of the examples that has me stumped and looking in the
documentation didn't help.
When I run the following code without the "import Data.Char" I get an
error that digitToInt is not defined.  When I put the import in I get a
large number of errors that weren't there before.
They were there, only ghci stopped on encountering an undefined name and 
didn't look for all errors then.

The errors are all of the same kind, 'lexeme', 'identifier', 'symbol' and 
'semi' - I hope, I haven't overlooked one - are named fields of a TokenParser 
and you try to apply lexeme to a Parser Int. If you insert 'lang' in the code 
after the abovementioned, the code will compile -- whether it'll do what is 
intended, I've no idea, I'd have to look at the sources to see what 
haskellStyle and makeTokenParser actually do (and of course I don't know what 
you want to have).

Hope, that's it,

Daniel
...
Can some one explain this to me?  How can I get this code to work?
--- Here is the code ---
...
module Expressionparser where
import Data.Char
import Text.ParserCombinators.Parsec
import Text.ParserCombinators.Parsec.Expr
import Text.ParserCombinators.Parsec.Token
import Text.ParserCombinators.Parsec.Language
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
runLex :: Show a => Parser a -> String -> IO()
runLex p
    = run (do{ whiteSpace lang
       ; x <- p
       ; eof
       ; return x
       }
    )
lang    = makeTokenParser
            (haskellStyle{ reservedNames = ["return","total"]})
expr = buildExpressionParser table factor  "expression"
table = [ [op "*" (*) AssocLeft, op "/" div AssocLeft]
  , [op "+" (+) AssocLeft, op "-" (-) AssocLeft]
  ]
    where
    op s f assoc
  = Infix (do{ symbol lang s; return f }  "operator") assoc
factor = parens lang expr
   <|> natural lang
    "simple expression"
test1   = do{ n <- natural lang
            ; do{ symbol lang "+"
                ; m <- natural lang
                ; return (n+m)
                }
            <|> return n
            }
-----------------------------------------------------------
price  :: Parser Int   -- this is the 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{ symbol "return"
      ; p <- price
      ; semi
      ; return (-p)
      }
    <|> do{ identifier
      ; p <- price
      ; semi
      ; return p
      }
     "produkt"
total = do{ p <- price
    ; symbol "total"
    ; return p
    }
--- end code ---
Here are the errors:
...
___         ___ _
  / _ \ /\  /\/ __(_)
 / /_\// /_/ / /  | |      GHC Interactive, version 6.2.2, for Haskell
98. / /_\\/ __  / /___| |      http://www.haskell.org/ghc/
\____/\/ /_/\____/|_|      Type :? for help.
Loading package base ... linking ... done.
Prelude> :l ~/expression-parser.hs
Compiling Expressionparser ( /home/greg//expression-parser.hs,
interpreted )
/home/greg//expression-parser.hs:59:
    Variable not in scope: `digitToInt'
Failed, modules loaded: none.
Prelude> :r
Compiling Expressionparser ( /home/greg//expression-parser.hs,
interpreted )
/home/greg//expression-parser.hs:51:
    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)))'
/home/greg//expression-parser.hs:67:
    Couldn't match `GenParser tok st' against `(->) String'
  Expected type: GenParser tok st t
  Inferred type: String -> CharParser st1 String
    Probable cause: `symbol' is applied to too few arguments in the call
  (symbol "return")
    In a 'do' expression: symbol "return"
/home/greg//expression-parser.hs:80:
    Couldn't match `GenParser Char ()' against `(->) String'
  Expected type: GenParser Char () t
  Inferred type: String -> CharParser st String
    Probable cause: `symbol' is applied to too few arguments in the call
  (symbol "total")
    In a 'do' expression: symbol "total"
Failed, modules loaded: none.
Prelude>

Re: [Haskell-cafe] a question about import and parsec...

Daniel Fischer