[Haskell-beginners] Applicative Parsec
byorgey at seas.upenn.edu
Fri Nov 6 11:49:05 EST 2009
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
> 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,
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.)
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