[Haskell-beginners] attoparsec: How to use the many1 combinator

21 Jun 2014

      I'm trying to do a first very simple example with attoparsec.

The file "test.txt" contains the line:

START 111 2222 333 END

The following code works and gives the result: Done "\n" 
["111","2222","333"]

{-# LANGUAGE OverloadedStrings #-}
import qualified Data.ByteString as BS
import Control.Applicative
import Data.Attoparsec.ByteString as P
main :: IO()
main = do
     bs <- BS.readFile "test.txt"
     parseTest pTest bs
pTest :: Parser [BS.ByteString]
pTest = do
     string "START"
     n1 <- pNumber
     n2 <- pNumber
     n3 <- pNumber
     string "END"
     return [n1, n2, n3]
pNumber :: Parser BS.ByteString
pNumber = do
     pSkipSpaces
     term <- P.takeWhile (\c -> c >= 0x31 && c <= 0x39)
     pSkipSpaces
     return term
pSkipSpaces :: Parser ()
pSkipSpaces = do
     P.takeWhile (\c -> c == 0x20)
     return ()

Unfortunately I must have exactly three numbers between START and END. 
To make this more flexible, I changed pTest:

pTest :: Parser [BS.ByteString]
pTest = do
     string "START"
     ns <- P.many1 $ pNumber <* (string "END")
     return ns

Now the program fails with: Fail "2222 333 END\n" [] "Failed reading: 
takeWith"
Why ??

Many thanks for your help!

[Haskell-beginners] attoparsec: How to use the many1 combinator

Roland Senn