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

Quiz — whenever and done

What does the following program print?

my @out;

react {
    whenever Supply.from-list(1, 2, 3) {
        @out.push($_);
        done if $_ == 2;
    }
    whenever Supply.from-list(10, 20) {
        @out.push($_);
    }
}

say @out;

1	[1 2]
0	[1 2 10 20]
0	[1 2 3 10 20]
0	[10 20]

The first whenever pushes 1, then 2 — and on 2 it calls done. The crucial point is that done ends the entire react block, not just its own whenever: the rest of the first supply (3) and the whole second supply (10, 20) are cut off before they are delivered. So @out holds just [1 2]. If done only silenced its own stream, you would have seen [1 2 10 20] — but one done stops all the reactions at once.

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Quiz — whenever and done

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