Builds a SignallingMapRef for effect F, initialized to the supplied value.
Builds a SignallingMapRef for effect F, initialized to the supplied value.
Update semantics for discrete are the same as SignallingRef, with one exception:
it cannot distinguish updates that remove a key (by setting its value to None).
More specifically: if you remove a key, this will only notify once per listener
i.e. setting it to None again will not trigger another update.
Furthermore, if a listener's last pull returned None, and by the time it pulls again the
current value is None, then it will not be notified regardless of any non-None updates
that may have happened between the pulls. This special semantic for None is necessary to
prevent memory leaks at keys with no values and no listeners.