Re: [Haskell-cafe] Parsec lookAhead with choice (<|>) doesn't work as expected?

2 May 2014

      Many thanks for your timely help.

Just tried:

nocPeekHeader = do
  st <- (lookAhead noc28hdr') <|>  (lookAhead noc25hdr')

noc28hdr' = try noc28hdr
noc25hdr' = trynoc25hdr

still fail with the same error:

--
(input1.txt)
SNOC ERROR: ERRLOG0 = 0x80030000

(input2.txt)
SNOC ERROR: ERRSTATUS = 0x00000001

Right (28,"SNOC")
Left "input2.txt" (line 1, column 2):
unexpected "S"
expecting "NOC ERROR: ERRLOG"
*Main>

On Fri, May 2, 2014 at 2:45 PM, Roman Cheplyaka  wrote:
...
Try wrapping the first parser in 'try'.
* Baojun Wang  [2014-05-02 14:32:06-0700]
...
Hi List,
There is my first email on haskell-cafe, I must admit haskell is a
amazing
language even though I found it hard to program.
I'm trying to use Parsec to parse a text file something like:
format 1)
HEADER
BODY
format 2)
BODY
BODY = many DATA_FIELDS
I must distinguish the format is either 1) or 2), because I need process
them differently. So I'm trying to use lookAhead & <|>, something like:
format1Header = do
  HEADER_parser1
  HEADER_parser2
  return ..
format2Header = do
  -- whatever
  return ..
headerVersion = do
  version <- (lookAhead format1Header) <|> (lookAhead format2Header)
  return version
Howerver if format1Header parser failed headerVersion just return failure
without trying format2Header parser.
Full source code is attached, you can load the source then just run
``main''
I searched seems similar issue had been reported:
https://groups.google.com/forum/#!topic/fa.haskell/NGjQqR05-TA
Is there anything I'm fundamentally wrong with lookAhead and <|> in
Parsec?
How can I get headerVersion works as expected?
Thank you
Baojun