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

[David McBride]

You have a couple problems here.

P.many1 $ pNumber <* (string "END") ... which is the same as ... P.many1
(pNumber <* string "END")

In other words it matches 111 END 222 END 333 END.  Try this:

ns <- (P.many1 pNumber) <* string "END"

This almost works, but when you actually run it you'll get an infinite
loop.  The reason is because pNumber if it doesn't find a match it will
never actually fail, therefore many1 will continue using it forever
attempting to find a match that will never occur.  The reason why it never
fails is that it is composed of combinators that never fail, pSkipSpaces
and itself are both composed entirely of takeWhiles, which if they don't
find a match they just continue on without doing anything.  If you make a
small change though:

term <- P.takeWhile1 (\c -> c >= 0x31 && c <= 0x39)

then it works fine.




On Sat, Jun 21, 2014 at 8:47 AM, Roland Senn <rsx at bluewin.ch> wrote:

> 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!
>
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