Learning Perl 6

Chapter 18. Supplies, Channels, and Promises

Supplies and channels provide ways to send data from one part of a program to another. A Supply is a direct line of communication from a source of data to possibly many consumers. A Channel allows any part of your program to add data to a queue that any part of your program can read.

A Promise allows code to run asynchronously (concurrently)—different bits of code can run in overlapping time frames. This is quite handy while employing either Supplys or Channels (or both).

Supplies

A Supplier emits a message to every Supply that has asked to receive its messages. This happens asynchronously; they do their work as your program does other things. You can process things in the background and handle the results as they come in rather than stopping the entire program to wait for all of the data. Other languages may call this “Publish–Subscribe” (or “PubSub”).

Here’s a useless example. Set up a Supplier and call .emit to send a message. Since you didn’t define any Supplys that message goes nowhere; it’s gone forever:

my $supplier = Supplier.new;
$supplier.emit: 3;

To receive that message ask the Supplier for a Supply (yes, the terminology is a bit thick) by calling .tap with a Block:

my $supplier = Supplier.new;
my $supply   = $supplier.Supply;
my $tap      = $supply.tap: { put "$^a * $^a = ", $^a**2 };
$supplier.emit: 3;

The Supply receives the 3 and passes that as the argument to the Block, which then outputs the message:

3 + 3 = 9

There are some useful builtin ...