[Haskell-cafe] Recursive attoparsec
Lian Hung Hon
hon.lianhung at gmail.com
Thu Oct 6 13:39:35 UTC 2016
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!
On 6 October 2016 at 01:05, Leonard Wörteler <leo at woerteler.de> wrote:
> Am 05.10.2016 um 14:32 schrieb Lian Hung Hon:
>> data Expression = ExpToken String | ExpAnd Expression Expression
>> How to write an attoparsec parser that parses this example:
>> "a" and "b" and "c"
>> 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
> 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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