[Haskell-cafe] Recursive attoparsec

Lian Hung Hon hon.lianhung at gmail.com
Thu Oct 6 13:39:35 UTC 2016


Dear guys,

Thanks! chainr and chainl are exactly what I was looking for. I did
something along the lines of

andParser = ExpAnd <$> ((stringParser <|> andParser) <* "and") <*>
(stringParser <|> andParser)

I can see now, why that wouldn't work!

Regards,
Hon

On 6 October 2016 at 01:05, Leonard W├Ârteler <leo at woerteler.de> wrote:

>
>
> Am 05.10.2016 um 14:32 schrieb Lian Hung Hon:
>
>> Given
>>
>> data Expression = ExpToken String | ExpAnd Expression Expression
>>
>> How to write an attoparsec parser that parses this example:
>>
>> "a" and "b" and "c"
>>
>> into
>>
>> ExpAnd (ExpToken "a") (ExpAnd (ExpToken "b") (ExpToken "c"))?
>>
>
> You can re-implement `chainr1` from Parsec as follows:
>
>     chainr1 :: Parser a -> Parser (a -> a -> a) -> Parser a
>     chainr1 p op = scan
>       where
>         scan   = p >>= rest
>         rest x = op <*> pure x <*> scan <|> return x
>
> Then you just plug in your parsers for variables and the `and` operator.
> Working example attached.
>
> -- Leo
>
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