On 26/07/12 13:58, Евгений Пермяков wrote:
As you can see, if I use select definition with Control.Applicative.<*>, I'll execute both l and r and the only choice will be, what result to drop. Both l and r, however, will be executed, and their side effects will take place. With select from my code only one action will be executed, depending on result of i, and only effects of one of actions (either l or r) will take place.
I realize that, and that is why I insisted on laws to formalize this. Your instance for IO is a special case of a function that works for any Monad: defaultEitherF :: (Functor f, Monad f) => f (a -> c) -> f (b -> c) -> f (Either a b) -> f c defaultEitherF ml mr mx = either (ml <$$>) (mr <$$>) =<< mx where (<$$>) :: Functor f => f (a -> b) -> a -> f b f <$$> x = ($ x) <$> f (the version of this function in my previous post was not correct)
I'm not sure, what categorical concept will correspond to this typeclass.
Well, this type class kind of corresponds to the functionality of ArrowChoice. I believe that class corresponds to a (symmetric) monoidal structure on the dual category. Plus a whole bunch of junk you get from its super classes. Twan