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

The react block

A react block is a place where reactions live. Inside it, a whenever says “whenever this supply emits a value, run this body”. The react block then waits until every supply it watches is finished:

my $sum = 0;

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

say $sum; # 6

The whenever body runs once for each value — 1, 2, 3 — adding it to $sum. The topic value is available as $_ (you can also name it with -> $v).

The key behaviour is that react blocks until its supplies are done. Only after the Supply.from-list has emitted all its values does the react block finish and the program move on to the say. That is why $sum is already 6 by the time it is printed: the react block waited for the whole stream.

This makes react a natural place to gather results from a stream: set up the reactions, let it run to completion, and use what you collected afterwards.

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