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

[Roman Cheplyaka]

Try wrapping the first parser in 'try'.

* Baojun Wang <wangbj at 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

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