In Concurrency§

See primary documentation in context for react

The react keyword introduces a block of code containing one or more whenever keywords to watch asynchronous events. The main difference between a supply block and a react block is that the code in a react block runs where it appears in the code flow, whereas a supply block has to be tapped before it does anything.

Another difference is that a supply block can be used without the whenever keyword, but a react block requires at least one whenever to be of any real use.

react {
    whenever Supply.interval(2) -> $v {
        say $v;
        done() if $v == 4;
    }
}

Here the whenever keyword uses .act to create a tap on the Supply from the provided block. The react block is exited when done() is called in one of the taps. Using last to exit the block would produce an error indicating that it's not really a loop construct.

An on-demand Supply can also be created from a list of values that will be emitted in turn, thus the first on-demand example could be written as:

react {
    whenever Supply.from-list(1..10) -> $v {
        say $v;
    }
}