Programming Crystal

Letting Modules Extend Themselves

The self. prefix for top-level methods can become a bit cumbersome. You can get rid of it by making a module extend itself:

working_with_modules/moral2.cr

module Moral
  extend self
  VERY_BAD = 0
  BAD      = 1

  def sin(badness)
    puts "Assessing the sin of #{badness}"
  end
end

You’ll see this common idiom in modules—for example, module Math uses it in its code. When applied, a module is used as a namespace and you can write Module.method, like Math.sin. Now if this looks like a class method to you, it is! Remember: A module is of type Class.

working_with_modules/namespaces.cr

require "./moral2"

y = Math.sin(Math::PI/4)                # => 0.70710678118654746
wrongdoing = Moral.sin(Moral::VERY_BAD) ...