[Haskell-beginners] Applicative Parsec

[Deniz Dogan]

2009/11/6 Brent Yorgey <byorgey at seas.upenn.edu>:
> 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
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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