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

[Roland Senn]

David,

Many thanks! Yes I had the infinite loop to0.

Regards
    Roland

Am 21.06.2014 15:33, schrieb 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 
> <mailto: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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