2009/11/6 Brent Yorgey :

On Fri, Nov 06, 2009 at 05:22:04PM +0100, Matthias Guedemann wrote:

...

I wrote a converter from a textfile format to a csv file. In order to get more familiar with Haskell I wrote it using Parsec. At first I had some difficulties and the monadic version seemed easier, but now it works as Applicative version and Parsec seems really straightforward.

For me, the Applicative version looks more functional and the monadic version looks more imperative.

Indeed; thta's a good way of putting it.  Applicative looks more functional since it corresponds to function application, but a special sort of function application that carries along some sort of "context".

...

It seems that Applicative is enough for Parsers (think I remember a citation somewhere), but does it have a real advantage?

The Monad interface gives strictly more power: it allows you to give names to intermediate results, and *decide what to do* later based on those intermediate results.  With Applicative, the actions that you perform must be fixed ahead of time.

For example, consider parsing a file which contains a positive integer, followed by that many letters.  For example,

 3xyz  12abcdefghijkl

are two instances of this format.  In order to parse this, a monadic interface is required, since the result of parsing the number must be used to decide how many things to parse after that.

However, for *many* purposes, an Applicative parsing interface is all you need.  And if Applicative is enough, it's usually nicer/more elegant than Monad. (And using the least powerful/most general thing that works for your purpose is usually good style anyway.)

-Brent _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

In Hughes' and Swierstra's paper "Polish Parsers, Step by Step" (2003) it says that Parsec's monadic interface can not be used for online production of results. I don't know why or whether this even is true anymore. -- Deniz Dogan

On Fri, Nov 06, 2009 at 07:20:11PM +0100, Matthias Guedemann wrote:

Hi Brent,

thanks for the illustrative example.

...

For example, consider parsing a file which contains a positive integer, followed by that many letters. For example,

3xyz 12abcdefghijkl

are two instances of this format. In order to parse this, a monadic interface is required, since the result of parsing the number must be used to decide how many things to parse after that.

I see, but as long as I want to parse context free grammars, it is sufficient?

Well, technically, if you want to be able to do any choice at all, you need Alternative in addition to Applicative (this is what provides the <|> choice operator). But intuitively, yes, I expect that you should be able to parse any context-free grammar using only Alternative. -Brent