Mutually Exclusive Alias Method And Prepend

Let us consider the following example:

1class Person
2 def greeting
3  puts "Hello"
4 end
5end
6
7class Person
8  def greeting_with_expression
9    puts "Hi,Hello"
10    greeting_without_expression
11  end
12
13  alias_method :greeting_without_expression, :greeting
14  alias_method :greeting, :greeting_with_expression
15end

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:

1module Greeting
2  def greeting
3    puts "!!! Hello"
4    super
5  end
6end
7
8Person.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 !!!