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

Live supplies in action

Because a live supply broadcasts to whatever taps are listening at the moment, several listeners can follow the same stream at once — and each is free to join or leave whenever it likes. Here two listeners watch a stream of events: a log that records everything, and an alert that joins later and leaves earlier.

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

my $log = $events.tap(-> $v { say "log:   $v" });    # first listener joins
$supplier.emit('start');

my $alert = $events.tap(-> $v { say "alert: $v" });  # a second listener joins
$supplier.emit('warning');
$supplier.emit('all clear');

$log.close;                                          # the first listener leaves
$supplier.emit('stop');

This prints:

log:   start
log:   warning
alert: warning
log:   all clear
alert: all clear
alert: stop

The lifecycle of the two taps lines up like this — 🟢 marks a tap joining, 🔴 a tap leaving, and ✅ each value a tap actually receives:

A blank cell means that tap was not listening when the value went out. Follow the timeline:

• .tap returns a Tap object — the handle you keep to control that one subscription.
• start is emitted while only log is listening, so only log sees it. This is the live-supply rule again: alert joined after start, and a live supply does not replay the past, so alert never sees it.
• warning and all clear are emitted while both are tapped, so each value reaches both — in the order the taps subscribed, log first, then alert.
• Calling $log.close leaves the supply. From then on log receives nothing, so the final stop reaches only alert.

Two things to take away. First, a single live supply can fan out to many independent taps at once. Second, each tap is its own subscription with its own lifetime: hold on to the Tap object that .tap returns, and you can end that one listener with .close without disturbing the others.

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