Revise FiberLocal

[jdegoes]

Copy-on-write semantics with manual or auto (fork-driven) propagation.

1. A FiberLocal is created.
2. Fiber A sets the value of this FiberLocal to 42.
3. Fiber A forks another fiber, Fiber B.
4. Fiber B reads the value of the FiberLocal and sees 42.
5. Fiber B sets the value of the FiberLocal to 3.
6. Fiber B reads the value of the FiberLocal and sees 3.
7. Fiber A reads the value of the FiberLocal and sees 42.

[jdegoes]

It's not necessary to make fork the primitive that does propagation. One can imagine a copyOnWrite primitive:

for {
  propagate <- fiberLocal.copyOnWrite
  fiber     <- (propagate *> child).fork
} yield ...

I don't know what the API would look like. But it could be something like this, and possibly used automatically in fork, as that seems the most common use case.

[jdegoes]

/cc @Kaishh

[iravid]

yes please!!

[jdegoes]

@iravid What issues would this solve for you?

Basically it would solve all mine, ever. 😆

[iravid]

contextual logging, using the same pool for submitted Spark computations, etc...