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