On-demand supplies

The data flow of supplies contains two parts—the supplier that emits data and the tap that receives it. Perl 6's reactive programming model is a thread-safe implementation of the Observer design pattern.

Let us create our first on-demand supply using the supply keyword:

supply {
    emit($_) for 'a'..'e';
}

The supply is here but it does not emit any data yet because there is no demand. You can easily see this if you add a print instruction to the loop:

supply {
    for 'a'..'e' {
        emit($_); 
        say "Emitted $_";
    }
}
sleep 2;

The program just silently quits after 2 seconds.

To make the supply generate data, we need to create a tap. The supply block returns a value of the Supply type, and you can call the tap method on it to pass the code that will be executed in response to the data emitted:

supply {
    emit($_) for 'a'..'e';
}.tap({
    .say;
});

This time, the program prints a few lines with the letters from a to e. Let us open a tap in our "debugger" program...