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 <roma@ro-che.info> wrote:

Try wrapping the first parser in 'try'.

* Baojun Wang <wangbj@gmail.com> [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