Include/Extend in Ruby Modules

I have always confused my self when to use include or extend when mixin methos of a class.

include() mixes methods into the instances of the base object.

extend() mixes methods into the base object itself.

For example,

module Color
  def green
    "I'm green"
  end
end

By including Greeter into SomeClass, we make it so that we can now call hello() on instances of SomeClass.

class SomeClass
  include Color
end

SomeClass.new.green #=> "I'm green"

Extending AnotherClass with Greeter would allow us to call the hello method directly at the class level, as in the example below.

class AnotherClass
  extend Color
end

AnotherClass.green #=> "I'm green"

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