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Some times you don’t need the overhead of creating an object with state. In these cases, a module can be used.

A module is very similar to a class (in fact, Module is Classes parent in the object hierarchy) - the main difference being that rather than using instance methods, we use class methods. Class methods start with self. and are directly called on a module. For example:

module Speaker
  def self.echo(something)
    "#{something} ... #{something}"
  end
end

Speaker.echo("Hello")   #=> "Hello ... Hello"

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Originally from Exercism ruby concepts