[Haskell-cafe] Best practices for Arrows?
Tom Ellis
tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk
Sat Jun 22 20:05:09 CEST 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 <tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> 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
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