Monadic parser vs. combinator parser
I will be writing a parser in Haskell and I wonder how to approach the problem. My first thought was to use monadic parser, e.g. like the one described by Hutton and Meijer in "Monadic Parsing in Haskell" functional pearl. But then I stumbled upon this: https://github.com/alephnullplex/cradle/tree/master/code/src/Lbach/Parser Monadic parser seems extremely verbose and not very straightforward compared to this one. I started to wonder whether I should use monadic parser for the sake of it being monadic or should I just go with the combinator approach? Any thoughts will be appreciated before I shoot myself in the foot :) Janek
Jan Stolarek
I will be writing a parser in Haskell and I wonder how to approach the problem. My first thought was to use monadic parser, e.g. like the one described by Hutton and Meijer in "Monadic Parsing in Haskell" functional pearl. But then I stumbled upon this:
https://github.com/alephnullplex/cradle/tree/master/code/src/Lbach/Parser
Monadic parser seems extremely verbose and not very straightforward compared to this one. I started to wonder whether I should use monadic parser for the sake of it being monadic or should I just go with the combinator approach? Any thoughts will be appreciated before I shoot myself in the foot :)
A monadic parser /is/ a combinator parser. The code you linked just doesn't go as far as wrapping it up with a newtype and providing a monad instance. Monadic parsers aren't verbose, because there is the applicative style. Let's rewrite this noisy example (assuming automatic backtracking): inParens c = do char '(' x <- c char ')' return x All monads are also applicative functors, which means that you can use applicative style to write this one more nicely: inParens c = char '(' *> c <* char ')' If your parser is also an IsString you could even write: inParens c = "(" *> c <* ")" If that's not nice and concise I don't know what is. =) Greets, Ertugrul -- Not to be or to be and (not to be or to be and (not to be or to be and (not to be or to be and ... that is the list monad.
On 30 January 2013 12:38, Ertugrul Söylemez
A monadic parser /is/ a combinator parser. The code you linked just doesn't go as far as wrapping it up with a newtype and providing a monad instance.
Further, (+>) in the linked example is monadic bind and `result` is `return`. The code looks more succinct than early Parser combinator libraries (like Hutton / Meijer) because it defines quite a few more combinators. Equivalents are available if you use say Parsec plus the usual applicative combinators.
Thanks for replies guys. I indeed didn't notice that there are monads and applicatives used in this parser. My thought that monadic parsers are more verbose came from Hutton's paper where the code is definitely less readable than in example I provided. There is one more thing that bothers me. It is easy to write a parser that returns Nothing when parsing fails. But I can't figure out a way to add meaningful error messages so that the user knows where did the parsing fail. I experimented with using Either so that I can use Left to pass error messages but this turned out to be inflexible and clutered the code. I will be greatful for any ideas. Janek
Jan Stolarek
Thanks for replies guys. I indeed didn't notice that there are monads and applicatives used in this parser. My thought that monadic parsers are more verbose came from Hutton's paper where the code is definitely less readable than in example I provided.
There is one more thing that bothers me. It is easy to write a parser that returns Nothing when parsing fails. But I can't figure out a way to add meaningful error messages so that the user knows where did the parsing fail. I experimented with using Either so that I can use Left to pass error messages but this turned out to be inflexible and clutered the code. I will be greatful for any ideas.
Remember that 'Either e' is also a monad. =)
Greets,
Ertugrul
--
Key-ID: E5DD8D11 "Ertugrul Soeylemez
Remember that 'Either e' is also a monad. =) I remember - this makes the change from Maybe to Either very easy :) Still I found that adding error message to every combinator and function ads a lot of boilerplate. Also, I experince
Dnia czwartek, 31 stycznia 2013, Ertugrul Söylemez napisał: problem with granularity of the messages, e.g. a message like "digit expected" is too low-level not helpful without telling user that the problem was in the incorect date format. Janek
On Jan 31, 2013, at 10:47 , Jan Stolarek
Thanks for replies guys. I indeed didn't notice that there are monads and applicatives used in this parser. My thought that monadic parsers are more verbose came from Hutton's paper where the code is definitely less readable than in example I provided.
There is one more thing that bothers me. It is easy to write a parser that returns Nothing when parsing fails. But I can't figure out a way to add meaningful error messages so that the user knows where did the parsing fail. I experimented with using Either so that I can use Left to pass error messages but this turned out to be inflexible and clutered the code. I will be greatful for any ideas.
Use the uu-parsinglib library, which provides error messages, repairs your errors and using its idioms definition you can even write: inParens c = iI '(' c ')' Ii I think you cannot get it shorter and with more functionality. Doaitse
Janek
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Dnia niedziela, 3 lutego 2013, Doaitse Swierstra napisał:
Use the uu-parsinglib library, which provides error messages, repairs your errors and using its idioms definition you can even write:
inParens c = iI '(' c ')' Ii
I think you cannot get it shorter and with more functionality.
Doaitse Thanks Doaitse. I know about your parsing libraries and at some point I will switch to using them (or Parsec). But for now I am more interested in educating myself so I'm trying to write things from scratch.
Janek
On Wed, Jan 30, 2013 at 1:21 PM, Jan Stolarek wrote:
I will be writing a parser in Haskell and I wonder how to approach the problem.
Utrecht University has a course that covers this, among other things. You might find the slides and lecture notes useful: http://www.cs.uu.nl/wiki/TC/CourseMaterials Regards, Sean
On 1/30/13 7:21 AM, Jan Stolarek wrote:
I will be writing a parser in Haskell and I wonder how to approach the problem. My first thought was to use monadic parser, e.g. like the one described by Hutton and Meijer in "Monadic Parsing in Haskell" functional pearl. But then I stumbled upon this:
https://github.com/alephnullplex/cradle/tree/master/code/src/Lbach/Parser
Monadic parser seems extremely verbose and not very straightforward compared to this one.
Psst, result :: a -> Parser a (+>) :: Parser a -> (a -> Parser b) -> Parser b cf. return :: Monad parser => a -> parser a (>>=) :: Monad parser => parser a -> (a -> parser b) -> parser b You just lose the nice do-notation is all. -- Live well, ~wren
participants (6)
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Doaitse Swierstra -
Ertugrul Söylemez -
Jan Stolarek -
Sean Leather -
Stephen Tetley -
wren ng thornton