Extended supervisor API #132

The supervised combinator lets one fork freely and have the runtime automatically clean up fibers when the inner block exits. I think we could potentially extend this API to have more control over things like unhandled exceptions.

Implementation-wise, the supervision context is essentially a MonadReader environment. When we fork anything, we pull out the supervision context and use that to track fibers. I think we could potentially reify this environment, giving authors control over which supervisors run which fibers, and also what happens when they exit abnormally.

As a baseline, I could imagine an API like:

data Supervisor eff

makeSupervisor :: forall eff. Aff eff (Supervisor eff)

-- We need a special global supervisor.
-- Equivalent to `ask`.
getCurrentSupervisor :: forall eff. Aff eff (Supervisor eff)

-- Equivalent to `local`
runWithSupervisor :: forall eff a. Supervisor eff -> Aff eff a -> Aff eff a

-- Tracks Fiber using provided supervisor
forkWithSupervisor :: forall eff a. Supervisor eff -> Aff eff a -> Aff eff (Fiber eff a)

-- Kills all pending Fibers
killAll :: forall eff. Error -> Supervisor eff -> Aff eff Unit

Which we could use to implement existing functionality. supervised would then be something like:

supervised aff = do
  sup <- makeSupervisor
  finally (killAll sup) $ runWithSupervisor sup aff

And fork would be something like:

fork aff = do
  sup <- getCurrentSupervisor
  forkWithSupervisor sup aff

Some possible ways to extend the API:

• Supervisor delegation: create a context extended from a parent context.
• Respond to unhandled exceptions.
• A way to associate some tag/state/process id with a Fiber.