Ronald Guida wrote:
I have read that Monad is stronger than Idiom because Monad lets me use the results of a computation to choose between the side effects of alternative future computations, while Idiom does not have this feature. Arrow does not have this feature either.
ArrowChoice has the feature that the sum type, Either, can be used to choose between alternative computations, including their side effects. I know that Monad is supposed to be stronger than ArrowChoice, but I have to ask, what exactly can Monad do that ArrowChoice can't do?
Monads are equivalent to ArrowApply, they allow you to use a computed *action*. For example: getMissileLauncher :: IO (String -> IO ()) notWithArrows = do launchMissiles <- getMissleLauncher launchMissiles "Russia" ArrowChoice is not enough to implement this function. But it is possible with ArrowApply, of which Kleisli IO is an instance. Twan