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

Supplies defined outside

Every whenever so far has created its supply right there in the header — whenever Supply.from-list(...). That reads well for a quick example, but real programs rarely have their event source on hand at exactly that spot. More often a supply is built somewhere else — returned by a function, produced by a Supplier, or handed to you by a socket or a timer — and arrives as a plain variable. A whenever is just as happy to watch a named supply:

my $sensors  = Supply.from-list(18, 21, 19);
my $messages = Supply.from-list('hi', 'bye');

react {
    whenever $sensors {
        say "temperature: $_";
    }
    whenever $messages {
        say "message: $_";
    }
}

This prints:

temperature: 18
temperature: 21
temperature: 19
message: hi
message: bye

So read whenever $sensors { … } as “whenever the $sensors supply — wherever it came from — emits a value, run this body”. The inline whenever Supply.from-list(...) you saw earlier is just the special case where the source happens to be created on the spot.

If this outer-block-with-inner-blocks shape reminds you of something, you are right — the next page compares it with given/when.

← Quiz — whenever and done | whenever vs when →

💪 Or jump directly to the exercises in this section.

Supplies defined outside

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