Course of Raku / Functional, concurrent, reactive, and web programming / Reactive programming / react and whenever

Two timers

Every react example so far has used Supply.from-list, which pushes all of its values the instant it is tapped. That is why each stream printed in full before the next one started — there was no waiting involved. Real event streams are not like that: their values arrive spread out over time, and a react block interleaves them as they come.

A timer is the simplest stream that behaves this way. Supply.interval($period) emits an increasing counter — 0, 1, 2, … — once every $period seconds. Run two of them at different rates inside one react, and their values weave together:

react {
    whenever Supply.interval(0.3) {
        say "tick $_";
    }
    whenever Supply.interval(0.5, 0.25) {
        say "TOCK $_";
    }
    whenever Promise.in(1.4) {
        done;
    }
}

A typical run prints:

tick 0
TOCK 0
tick 1
tick 2
TOCK 1
tick 3
tick 4
TOCK 2

The fast tick fires every 0.3 seconds; the slower TOCK every 0.5, starting a quarter-second in (the second argument to interval is that initial delay). Neither waits for the other — the react block just runs whichever whenever body goes with the value that has arrived, so the two streams intersect instead of taking turns.

Because timers never finish on their own, a third whenever provides the exit: Promise.in(1.4) is kept after 1.4 seconds, and its body calls done to end the react block. Without it, the react would tick and tock forever.

The exact interleaving depends on timing, so a later run may order the lines a little differently — but that is the nature of reacting to live events: you respond to each as it happens, whenever it happens.

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Two timers

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