Mutually Exclusive Alias Method And Prepend

Module#alias_method was one of the earliest solution to overwrite a method in ruby.With an alias method in place you can redefine a method to anything you want and call the old method using the alias_method

Let us consider the following example

class Person
 def greeting
  puts "Hello"
 end
end

class Person
  def greeting_with_expression
    puts "Hi,Hello"
    greeting_without_expression
  end

  alias_method :greeting_without_expression, :greeting
  alias_method :greeting, :greeting_with_expression
end

In the example above Person#greeting is overridden using alias method.greeting_without_expression calls the actual greeting method.

With the advent of Module#prepend in Ruby 2.0 It allows you override a method in a class with a method from a module, and still access the class’s implementation with super

module Greeting
  def greeting
    puts "!!! Hello"
    super
  end
end

Person.prepend(Greeting)

Greeting module is prepended to Person. And now when you call

Person.new.greeting

You will end up with

SystemStackError: stack level too deep

What happened on the background is that Greeting#greeting was called at first then it calls its super_method(Person#greeting_with_expression). At the end of execution of greeting_with_expression it calls the greeting_without_expression method which again calls Greeting#greeting due to method lookup.This creates a deadlock situation and results in stack level too deep error.

Learning from the above example is that you will not be able to combine Module#prepend and Module#alias_method.You can follow only one of these method if you want to overwrite a method at multiple levels.

Lookup for alias method and prepend when you come across stack level too deep error next time.Happy Coding !!!