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

[Mateusz Kowalczyk]

On 05/03/2014 01:37 AM, Baojun Wang wrote:
> Thanks, problem fixed by adding try, the error turns out to be something
> else :)
> 

I'd like to note that Attoparsec backtracks by default, might be worth
looking into.

> 
> On Fri, May 2, 2014 at 2:53 PM, Baojun Wang <wangbj at gmail.com> wrote:
> 
>> 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 at ro-che.info> wrote:
>>
>>> 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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-- 
Mateusz K.