Programming Ruby 3.3

Chapter 18Typed Ruby

When you see a variable in your code, it’s useful to know what values can be assigned to that variable without the code breaking. For example, you might have the following Ruby method:

def mystery_method(x)
  x * 3
end

You’d likely expect that x should be a number. But it’s also completely valid Ruby for x to be a string ("a" * 3 resolves to “aaa”) or an array ([:a] * 3 resolves to [:a, :a, :a]).

Let’s say that this method is in your code, and over time people call mystery_method with strings, integers, floating-point numbers, and so on, until somebody changes the method and inadvertently changes what variables it’ll accept. Here’s an example:

def mystery_method(x)
  x.abs * 3
end

Now all of those string ...