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

Live supplies

A live supply broadcasts values as they occur to whatever taps are listening at that moment. You create one with a Supplier: the supplier is the sending end, and its .Supply is the receiving end that others tap.

my $supplier = Supplier.new;
my $supply   = $supplier.Supply;

my @got;
$supply.tap(-> $v { @got.push($v) });

$supplier.emit(1);
$supplier.emit(2);

say @got; # [1 2]

Here .emit pushes a value into the live supply, and every current tap receives it. The tap was in place before the emissions, so it captured both values.

The crucial difference from an on-demand supply is timing: a live supply does not remember past values. A tap added after an emit would miss whatever was emitted before it subscribed. Live supplies are the right model for real events — clicks, messages, sensor readings — where “what is happening now” matters and the past is gone.

In short: use an on-demand supply to replay a fixed sequence to each subscriber, and a live supply (via a Supplier) to broadcast events as they happen.

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