[Haskell-cafe] Re: request for code review

Shannon -jj Behrens jjinux at gmail.com
Tue Mar 14 14:58:55 EST 2006

On 3/14/06, Benjamin Franksen <benjamin.franksen at bessy.de> wrote:
> On Tuesday 14 March 2006 14:46, Pete Chown wrote:
> > Shannon -jj Behrens wrote:
> > > Arrows looks like a replacement for monads.  Are you saying
> > > I should drop my use of the State monad?  If so, why?  I like the
> > > readability of the do syntax.
> >
> > Okay, now it's my turn to ask a question. :-) I've read about arrows,
> > and while I think I see what they do, I'm not sure why they are seen
> > as so special that they even get new syntax.  This question of
> > Shannon's is exactly the point I struggle with.  I can see that the
> > arrow operators might be useful with functions, but are they useful
> > for other things too?
> Yes, http://www.haskell.org/arrows/biblio.html lists a number of papers
> describing non-trivial applications of Arrows, that is, Arrows other
> than (->). I found the exposition in
> http://www.haskell.org/yale/papers/oxford02/ to be quite readable.
> > For example, as monads are one kind of arrow,
> > I thought I would make some of the I/O functions into arrows and see
> > what happened.  The result was pretty much the same as using the
> > monad, except slightly less convenient.
> You can write monadic code without ever using the syntax sugar, and get
> along. However, do-notation is convenient. OTOH, I am told that
> programming with Arrows is really quite inconvenient w/o the syntax
> sugar.

Well, forgive me for my newbie-ness:

o How important is it that I switch from using the State monad to using arrows?
o How important is it that I switch from using |> or $ to using
arrows?  (It seems that using arrows just to replace |> or $ is like
using a sledge hammer to drive a thumb tack.)
o How much will this increase the "conceptual complexity" of my
program--i.e. how much time am I going to have to spend explaining it
in my article?
o How much will this improve the readability or decrease the amount of
code in my program?


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