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Using lexeme in parsec

I do not understand how to use the lexeme function

I saw the above question, but I still do not understand.

The example in the documentation , for example, also does not work.

mainParser  = do{ whiteSpace
                ; ds <- many (lexeme digit)
                ; eof
                ; return (sum ds)
                }
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2 answers

Disclaimer: I have not used it yet Parsec. This lexemeis a field GenTokenParser s u m. If you check its type in GHCi, you will get

lexeme :: GenTokenParser s u m -> ParsecT s u m a -> ParsecT s u m a

Therefore, you already need a common parser parser, which you can create with makeTokenParser. The latter is of type:

makeTokenParser
  :: Stream s m Char =>
     Text.Parsec.Token.GenLanguageDef s u m
     -> Text.Parsec.Token.GenTokenParser s u m

. , emptyDef Text.Parsec.Language. , whiteSpace GenTokenParser. , ds :: [Char], digitToInt Data.Char, :

import Text.Parsec
import Text.Parsec.Token (lexeme, makeTokenParser, whiteSpace)
import Text.Parsec.Language (emptyDef)
import Data.Char (digitToInt)

lexer = makeTokenParser emptyDef

mainParser  = do{ whiteSpace lexer
     ; ds <- many (lexeme lexer digit)
     ; eof
     ; return (sum . map digitToInt $ ds)
     }

main = do
  putStrLn "Please give some digits (whitespaces are ignored)"
  line <- getLine
  case parse mainParser "" line of
    Right n -> putStrLn $ "Sum of digits is " ++ show n
    Left  _ -> putStrLn $ "Couldn't parse your line"

:

*Main> :main
Please give some digits
7 8 91 72 3945 01 92
Sum of digits is 67

*Main> :main
Please give some digits
abc 1
Couldn't parse your line
+5

: Haskell, .

import Text.Parsec
import qualified Text.Parsec.Token as T
import Text.Parsec.String ( Parser )
import Text.Parsec.Language (haskellDef)

lexer = T.makeTokenParser haskellDef

whiteSpace :: Parser ()
whiteSpace = T.whiteSpace lexer


lexeme = T.lexeme lexer

mainParser  = do whiteSpace
                 ds <- many digit
                 eof
                 return ds

.

Mukeshs-MacBook-Pro:Compilers mukeshtiwari$ ghci stmp.hs 
GHCi, version 7.6.1: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Main             ( stmp.hs, interpreted )
Ok, modules loaded: Main.
*Main> parse mainParser "" "1" 
Loading package array-0.4.0.1 ... linking ... done.
Loading package deepseq-1.3.0.1 ... linking ... done.
Loading package bytestring-0.10.0.0 ... linking ... done.
Loading package transformers-0.3.0.0 ... linking ... done.
Loading package mtl-2.1.2 ... linking ... done.
Loading package text-0.11.2.3 ... linking ... done.
Loading package parsec-3.1.3 ... linking ... done.
Right "1"
*Main> parse mainParser "" "12"
Right "12"
*Main> parse mainParser "" "123"
Right "123"
*Main> parse mainParser "" "      123"
Right "123"
*Main> parse mainParser "" "  123"
Right "123"
*Main> parse mainParser "" "     123"
Right "123"

. .

*Main> parse mainParser "" "123 "
Left (line 1, column 4):
unexpected ' '
expecting digit or end of input

Oops! - . ? , , , , ? whiteSpace, ( ), , , -, . ( ) eof, . , ? , whiteSpace , (Ignore < * ).

import Text.Parsec
import qualified Text.Parsec.Token as T
import Text.Parsec.String ( Parser )
import Text.Parsec.Language (haskellDef)
import Control.Applicative ( (<*) )
lexer = T.makeTokenParser haskellDef

whiteSpace :: Parser ()
whiteSpace = T.whiteSpace lexer

lexeme = T.lexeme lexer

mainParser  = do whiteSpace
                 ds <- many  ( digit <* whiteSpace )
                 eof
                 return  ds


*Main> parse mainParser ""   "    31312 "
Right "31312"
*Main> parse mainParser ""   "    3131 2 "
Right "31312"
*Main> parse mainParser ""   "    313 1 2 "
Right "31312"
*Main> parse mainParser ""   "    3 1 3 1 2 "
Right "31312"
*Main> parse mainParser ""   "    31 3 1 2 "
Right "31312"

. , . whiteSpace, ( (digit < * whiteSpace)). , whiteSpace , . lexeme, lexeme p p whiteSpace, lexeme , .

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