Section speaks about a merge operation in FRP streams processing (Sodium library is used). Book shows a below diagram of streams combination, and says that when event enters FRP logic through a stream, it causes a cascade of state changes that happen in a transactional context, so all changes are atomic.
Streams of events - sDeselect, sSelect (see 2 events: "+" and "-") are originating from UI controls, since they happen within the same FRP transaction their carried events are considered simultaneous. Then book says
The merge implementation has to store the events in temporary storage until a time when it knows it won’t receive any more input. Then it outputs an event: if it received more than one, it uses the supplied function to combine them; otherwise, it outputs the one event it received.
Question: When it is a time when "no more input will come"? How merge function knows this moment? Is it simply the time it gets a value from the 2nd incoming stream on a given diagram or i'm missing smth? Can you illustrate it with a better streams example?
The way Sodium does this is to assign rank numbers to the structure of the directed graph of FRP logic held in memory, in such a way that if B depends on A, then B's rank will be higher than A's. (Cycles are broken in the graph traversal that assigns these ranks.) These numbers are then used as the priorities in a priority queue with low rank values processed first.
During event processing, when the priority queue contains nothing lower than the rank of the merge, then it is known that there can be no more input data for the merge, and it will output a value.