Creating class level methods in a module in Ruby

Question

Why doesn't this work?

module Greeter
  def self.greet
    puts "anyang"
  end
end

Greeter.greet # => anyang

class KoreanKid
  include Greeter
  greet
end

# !> NameError : undefined local variable or method `greet' for KoreanKid:Class

KoreanKid.greet

# !> NoMethodError : undefined method `greet' for KoreanKid:Class

When I call greet right inside the KoreanKid class, that's just simply calling a class method right? That's the same thing as KoreanKid.greet right? Why doesn't the above work?

In my module, I'll have a mix of class methods and instance methods... how do I get both to work cleanly?

ndnenkov · Accepted Answer · 2017-03-21 15:05:14Z

1

Kernel#include adds the existing methods in the module as instance methods of the class. To add class methods, you have to use Kernel#extend:

module Foo
  def bar
    42
  end
end

class Baz
  extend Foo
end

Baz.bar # => 42

Note that the methods that we extended were instance methods in the original module.

A popular way to do both is to use the Module#included hook to also extend:

module Foo
  def bar
    :wut
  end

  def self.included(target)
    target.extend ClassMethods
  end

  module ClassMethods
    def baz
      :indeed
    end
  end
end

class Test
  include Foo
end

Test.new.bar # => :wut
Test.baz     # => :indeed

answered Mar 21, 2017 at 15:05

ndnenkov

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