Let's say I have a Future monad, so Future a represents an a that may or may not yet be available. Then sequenceA automatically gives me the semantics "wait for all of these futures to be ready and give me all their values":

sequenceA :: [Future a] -> Future [a]

This is something like a logical AND, since it doesn't become ready until all the inputs are ready. Carrying that metaphor further, we could define the logical OR: "become ready with the value of the first input that becomes ready".

firstReady: [Future a] -> Future a

Does this metaphore generalize to other monads/traversables? Is there a standard name for this operation?

CodePudding user response：

The async package exposes the Concurrently newtype which has an Alternative instance that gives rise to the "return the first thing that finishes" behavior.

The firstReady function is called asum :: [Concurrently a] -> Concurrently a.

But note that Concurrently doesn't have a monad instance, because it wouldn't be possible to satisfy the law:

m1 <*> m2 = m1 >>= (\x1 -> m2 >>= (\x2 -> return (x1 x2)))

(The <*> runs both its arguments at the same time but the >>= is forced to run its first argument fully before it can start running its second argument.)