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

whenever

A single react block can hold several whenever blocks, each watching a different supply. The block reacts to whichever supply emits next, running the matching body:

my $sum = 0;

react {
    whenever Supply.from-list(1, 2, 3) {
        $sum += $_;
    }
    whenever Supply.from-list(10, 20) {
        $sum += $_;
    }
}

say $sum; # 36

Both supplies are watched at once. Their values — 1, 2, 3 and 10, 20 — all flow into $sum, which ends at 36. The react block finishes only when both supplies are done.

Each whenever reacts independently, so this is how you combine streams: one whenever per source, all inside the same react. A real program might watch a network connection in one whenever and a timer in another, responding to each as its events arrive.

Within a whenever body you have the emitted value (as $_ or a named parameter) and can do anything with it — update state, emit to another supply, or even close the react block early with done.

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