Send Beginners mailing list submissions to
[email protected]
To subscribe or unsubscribe via the World Wide Web, visit
http://www.haskell.org/mailman/listinfo/beginners
or, via email, send a message with subject or body 'help' to
[email protected]
You can reach the person managing the list at
[email protected]
When replying, please edit your Subject line so it is more specific
than "Re: Contents of Beginners digest..."
Today's Topics:
1. attoparsec: How to use the many1 combinator (Roland Senn)
2. Re: attoparsec: How to use the many1 combinator (David McBride)
3. Re: attoparsec: How to use the many1 combinator (Roland Senn)
----------------------------------------------------------------------
Message: 1
Date: Sat, 21 Jun 2014 14:47:47 +0200
From: Roland Senn <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] attoparsec: How to use the many1
combinator
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-15"; Format="flowed"
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!
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Test.hs
Type: text/x-haskell
Size: 589 bytes
Desc: not available
URL:
<http://www.haskell.org/pipermail/beginners/attachments/20140621/3be9d117/attachment-0001.hs>
-------------- next part --------------
START 111 2222 333 END
------------------------------
Message: 2
Date: Sat, 21 Jun 2014 09:33:54 -0400
From: David McBride <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] attoparsec: How to use the many1
combinator
Message-ID:
<can+tr43yns3nwmtr_gdl9w5-gom3jvvjm6ro7kqby0asmtn...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
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 <[email protected]> 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!
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://www.haskell.org/pipermail/beginners/attachments/20140621/f10c0ea8/attachment-0001.html>
------------------------------
Message: 3
Date: Sat, 21 Jun 2014 15:51:20 +0200
From: Roland Senn <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] attoparsec: How to use the many1
combinator
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"; Format="flowed"
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 <[email protected]
> <mailto:[email protected]>> 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!
>
> _______________________________________________
> Beginners mailing list
> [email protected] <mailto:[email protected]>
> http://www.haskell.org/mailman/listinfo/beginners
>
>
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://www.haskell.org/pipermail/beginners/attachments/20140621/beb15b8b/attachment-0001.html>
------------------------------
Subject: Digest Footer
_______________________________________________
Beginners mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/beginners
------------------------------
End of Beginners Digest, Vol 72, Issue 16
*****************************************
