rget(r, @optional timeout, default = false)
Return the object bound to the reactive variable r.
If timeout is not specified, rget will wait till the reactive variable is bound by another task.
If timeout is specified and the variable is not bound within it runs-out, rget will return
the value of default. If there are no other tasks running this function will raise
a "Deadlock detected" error.
The ?r syntax has the same semantics as rget with no
timeout specified.
let x = ?
!{task_sleep(5); ?x = 100}
rget(x, 2) // wait for 2 seconds
// false
rget(x, 4) // wait for another 4 seconds, `x` should be bound by now.
// 100