From type/Supply

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method tap(Supply:D: &emit = -> $ { },
        :&done = -> {},
        :&quit = -> $ex { $ex.throw },
        :&tap = -> $ {} )

Creates a new tap (a kind of subscription if you will), in addition to all existing taps. The first positional argument is a piece of code that will be called when a new value becomes available through the emit call.

The &done callback can be called in a number of cases: if a supply block is being tapped, when a done routine is reached; if a supply block is being tapped, it will be automatically triggered if the supply block reaches the end; if the done method is called on the parent Supplier (in the case of a supply block, if there are multiple Suppliers referenced by whenever, they must all have their done method invoked for this to trigger the &done callback of the tap as the block will then reach its end).

The &quit callback is called if the tap is on a supply block which exits with an error. It is also called if the quit method is invoked on the parent Supplier (in the case of a supply block any one Supplier quitting with an uncaught exception will call the &quit callback as the block will exit with an error). The error is passed as a parameter to the callback.

The &tap callback is called once the Tap object is created, which is passed as a parameter to the callback. The callback is called ahead of emit/done/quit, providing a reliable way to get the Tap object. One case where this is useful is when the Supply begins emitting values synchronously, since the call to .tap won't return the Tap object until it is done emitting, preventing it from being stopped if needed.

Method tap returns an object of type Tap, on which you can call the close method to cancel the subscription.

my $s = Supply.from-list(0 .. 5);
my $t = $s.tap(-> $v { say $v }, done => { say "no more ticks" });

Produces:

0
1
2
3
4
5
no more ticks