Map-reduce is a common way of solving problems in functional programming. You have a list of items, you iterate through them to process them, and then you take the set and summarize them. We call this map-reduce because the iteration step is mapping values to new values and the summarize step reduces the number of values.
In Raku, map-reduce is a common programming pattern:
my $fibonacci = (1, 1, * + * .
Let’s consider now how to solve a big problem with concurrency. If you have an algorithmic problem with lots of data that needs to be processed, you want to maximize the amount of load you can place on the available CPU cores to process it as quickly as possible. To demonstrate how we can go about this in Raku, we will consider Conway’s Game of Life, played on an effectively infinite game board.
What’s faster than locks? Compare-and-swap. Modern CPUs have multiple cores. As such, all modern CPUs must have tools for performing absolutely atomic operations to allow those multiple cores to work together. One of those operations is the compare-and-swap or cas operation.
In the abstract, a cas operation takes three arguments, a variable to modify, a given value, and a new value. The variable is set to the new value modified only if the current value held by the variable is equal to the given value.
When writing concurrent code in Raku, we want to avoid sharing data between tasks. This is because code that shares no data is automatically safe and doesn’t have to worry about interdependencies with other code. So, when you can, you should do your work through Supply, Promise, and Channel objects that are then synchronized together by a central thread. That way, all the state changes are safe.
This is not always practical, though.
The react block in Raku is the primary means of re-synchronizing asynchronous coding activity. Using it, you can easily pull together promises, supplies, and channels to make a coherent whole of your program or a subsystem.
A react block itself can run any code you want plus one or more whenever blocks. The code in the block will run once and the block will exit either when a done subroutine is called or when all the objects associated with whenever blocks are finished (i.
A large number of concurrency-oriented coding in Raku depends on the use of a Scheduler. Many async operations depend on the default scheduler created by the VM at the start of runtime. You can access this via the dynamic variable named $*SCHEDULER.
The most important feature of a Scheduler is the .cue method. Calling that method with a code reference will schedule the work for execution. The type of scheduler will determine what exactly that means.
Warning! We are delving into the inner depths of Raku now. Threads are a low-level API and should be avoided by almost all applications. However, if your particular application needs direct Thread access, it is here for you.1
Use of the Thread class in Raku is straight-forward and looks very similar to what you would expect if you are familiar with threading tools in other languages:
my $t = Thread.start: name => 'Background task', :app_lifetime, sub { if rand > 0.
A Channel, in Raku, is an asynchronous queue of data. You can feed data in to one end of the queue and receive data at the other end safely, even when multiple threads are involved.
Let’s consider a variant of the Dining Philosopher Problem: We have five philosophers eating soup at the table. They do not talk to one another because they are too busy thinking about philosophy. However, there are only 2 spoons.
When you have a stream of data flowing through your Raku application that needs to be accessed safely among threads, you want a Supply. Today we’re going to discuss one particular way of working with supplies, the supply block. If you are familiar with sequences, aka Seq objects, a supply block works in a very similar fashion, but lets you pull work as it arrives and easily do something else in the meantime.
In Raku, Promises represent the simplest of the high-level language features for communicating between asynchronous tasks. They are very much like promises between people. For example, I might promise my son I will help him with his school work. I keep that promise when I help him. Or I break that promise if, for any reason, I fail to help him. The same is true of a Promise in Raku. A Promise to return a value is kept when that value arrives.
