Course of Raku / Functional, concurrent, reactive, and web programming / Reactive programming / Live and on-demand supplies

On-demand supplies

An on-demand supply produces its values anew for every tap. Supply.from-list is on-demand: each tap gets the whole sequence from the start, independently of any other tap.

my $s = Supply.from-list(1, 2, 3);

my @a;
my @b;
$s.tap(-> $v { @a.push($v) });
$s.tap(-> $v { @b.push($v) });

say @a; # [1 2 3]
say @b; # [1 2 3]

Both taps received all three values. The supply did not “use up” its values on the first tap; it simply ran again for the second. This is the behaviour you want when a supply represents a fixed source — a list, a file, a query — that every subscriber should see in full.

On-demand is the default for supplies built from existing data, because each consumer naturally expects the complete sequence. The other kind, a live supply, behaves quite differently, as the next topic shows.

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