On Mon, Feb 11, 2013 at 09:32:25AM +0100, Petr Pudlák wrote:
While the implementation of Applicative can be defined without actually using `delay`:
newtype ArrowApp a b c = ArrowApp (a b c)
instance Arrow a => Functor (ArrowApp a b) where fmap f (ArrowApp a) = ArrowApp (a >>^ f) instance ArrowDelay a => Applicative (ArrowApp a b) where pure x = ArrowApp $ arr (const x) (ArrowApp af) <*> (ArrowApp ax) = ArrowApp $ (af &&& ax) >>^ uncurry ($)
I believe it only satisfies the laws only if the arrow satisfies delay/force laws.
This is a reader, which always satisfies the applicative laws. What ArrowDelay does is pick out the arrows that are equivalent to the static arrow, i.e. F(b->c), of some applicative functor F.