Crystal by example: Modules

Modules in Crystal are used to organize code and share functionality. They can contain methods, constants, and types, but they cannot be instantiated.

Using modules as namespaces is simple—use the :: operator to access what is inside the module.

include adds the module’s methods as instance methods to the class or struct that includes it, and extend adds the module’s methods as class methods, tied to the class or struct itself.

Inside the module, self can be used to create methods that belong to the module itself. Modules do not allow inheritance, but they can be included in multiple classes or structs to reuse code.

module Constants
  PI  = 3.145
  TAU = PI * 2
  E   = 2.718
end

puts Constants::PI

module Greet
  def hello
    puts "Hello!"
  end
end

class Person
  include Greet
end

s = Person.new
s.hello

module MathHelpers
  def self.square(x)
    x * x
  end
end

puts MathHelpers.square(5)

module Trackable
  def track
    puts "Tracking #{self}"
  end
end

class Car
  include Trackable
end

class Bike
  include Trackable
end

Car.new.track
Bike.new.track

$ crystal run modules.cr

3.145
Hello!
25
Tracking #<Car:0x23c4dc96fb0>
Tracking #<Bike:0x23c4dc96fa0>