On 26/07/12 12:40, Евгений Пермяков wrote:
class Applicative f => Actuative f where -- | select computation conditionally . Side effects of only one two alternative take place select :: f (Either a b) -- ^ selector -> f (a -> c) -- ^ first alternative -> f (b -> c) -- ^ second alternative -> f c
Can't you already define this function in terms of Applicative itself? I.e. select xs fs gs = sel <$> xs <*> fs <*> gs where sel (Left a) f _ = f a sel (Right b) _ g = g b I assume that your intent is that `select` behaves differently from the one I defined here. But you need to specify in what way. Suppose it should work like if-then-else. Then you would perhaps have these laws: select (Left <$> x) f g = f <$> x select (fmap swapEither x) f g = select x g f I think this is a useful class to have, and I would support adding something like it to the standard library. Perhaps the arguments should be swapped to the same order as either, to give class Functor f => Selective f where eitherF :: f (a -> c) -> f (b -> c) -> f (Either a b) -> f c The laws would then be: eitherF f g . fmap swapEither = eitherF g f eitherF f g . fmap Left = f eitherF f g . fmap Right = g -- follows from the other two laws every Monad is an instance via defaultEitherF ls rs xs = either ls rs =<< xs Twan