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

Tapping a supply

To subscribe to a supply, you tap it: you give .tap a block, and that block runs for every value the supply emits. The simplest supply to experiment with is Supply.from-list, which emits the values of a list:

my $s = Supply.from-list(1, 2, 3);
$s.tap(-> $v { say $v });

This prints:

1
2
3

The block -> $v { say $v } is the tap. The supply pushes each value into it in turn, so the block runs three times, once per value.

The tap can do anything with the value, including accumulate a result:

my $total = 0;
Supply.from-list(1, 2, 3).tap(-> $v { $total += $v });
say $total; # 6

Here every emitted value is added to $total, which ends at 6. Tapping is the basic act of reactive programming: connect a piece of behaviour to a stream, and let the stream drive it.

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