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