Re: [Haskell-cafe] Best practices for Arrows?

22 Jun 2013

      Hi Ertugul.  Thanks for taking the time to write me an in-depth reply!  I
have a few comments and a question.

On Sat, Jun 22, 2013 at 03:36:15PM +0200, Ertugrul Söylemez wrote:
...
Tom Ellis  wrote:
...
Are there any best-practices I should be aware of with Arrows?  Or is
it just a case of getting on with it?
The best practice is probably to avoid them.  If your type is a monad,
there is little reason to use the awkward arrow interface.
Unfortunately my type doesn't have a Monad instance.
...
In most cases when you expose an `Arrow` interface you can also expose a
`Category`+`Applicative` interface, which is pretty much equivalent
(except for a few extra laws):
proc x -> do
        y1 <- a1 -< x
        y2 <- a2 -< x
        id -< x + y1 + y2^2
Is equivalent to:
liftA3 (\x y1 y2 -> x + y1 + y2^2) id a1 a2
Yes, I can see how that would be useful.  My question is: are you talking
about this Applicative instance:

data MyArr a b = ...

instance Arrow MyArr where
    ...

instance Functor (MyArr a) where
  fmap f = (arr f <<<)

instance Applicative (MyArr a) where
  pure = arr . const
  f <*> g = arr (uncurry ($)) <<< (f &&& g)
...
If the interface is not under your control, make yourself comfortable
with the complete arrow syntax, most notably how it handles operators,
combinators and the `(| banana bracket notation |)`.  This is very
valuable information.
Interesting.  I hadn't noticed the `(| banana bracket notation |)` on the
GHC Arrows page[1] before, but just saw it when I went back to check.
...
Try to separate individual computations as much as possible and compose
using `(.)` (or `(<<<)`/`(>>>)` if you prefer).  This makes your code
much more readable:
Yes, agreed.  I'm a strong proponent of using (.) for functions and (<=<)
when dealing with Monads.
...
There is one case where the arrow notation is really indispensable:
value recursion via `ArrowLoop`:
[...]
I think I will be able to make my Arrow an ArrowLoop, but I haven't
checked.

Thanks again,

Tom

[1] http://www.haskell.org/ghc/docs/latest/html/users_guide/arrow-notation.html