Add deferred/alt

[dm3]

This adds a deferred/alt combinator. Naming mirrors the core.async/alt.. functions. Also alt has the same length as zip, its dual. I find it aesthetically pleasing :)

I find it occasionally useful. Would you consider pulling into Manifold?

[danielcompton]

core.async/alt randomises the order that alt's are chosen in if several are available, unless :default is provided also. Randomising the order would probably be good here too?

[dm3]

@danielcompton could you please explain why randomization is useful in this case? I can't find any rationalization for why Core.Async does that.

[danielcompton]

Not sure if I can put it into words well, but the intuition is to avoid accidentally ordering things which shouldn't be ordered, or depending on the order that ports are declared in an alt.

Say you were testing with dummy data and you filled all your channels up, then ran the alt. If it took the first every time then you may miss subtle concurrency bugs that would occur in production.

[dm3]

This makes sense. I was thinking along the same lines. I see two options then:

• rename the function to deferred/any and match the CompletableFuture#anyOf which doesn't randomize but also doesn't specify anything in the docs.
• keep it as deferred/alt and randomize to match the Core.Async intuition

I have no strong feelings either way.

[danielcompton]

Should this pick a random ordering?

[ztellman]

Hi, I'm sorry for not responding to this sooner. I think I'd prefer the randomized approach, and alt as the name.

[dm3]

Randomized now.

I used the random-array-filling technique from core.async (except the storing Random in a thread local part). Not sure if that requires a Copyright notice in the sources and whether you'd like to go with that or use another method.

[ztellman]

Sorry for the delay in merging this, I'll be cutting a release soon.