Defining methods dynamically

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With Ruby you can modify the structure of the program in execution time. One way to do it, is by defining methods dynamically using the method method_missing.

Let’s say that we want to be able to test if a number is greater than other number with the syntax 777.is_greater_than_123?.

# open Numeric class
class Numeric
  # override `method_missing`
  def method_missing(method_name,*args)
    # test if the method_name matches the syntax we want
    if method_name.to_s.match /^is_greater_than_(\d+)\?$/
      # capture the number in the method_name
      the_other_number = $1.to_i
      # return whether the number is greater than the other number or not
      self > the_other_number
    else
      # if the method_name doesn't match what we want, let the previous definition of `method_missing` handle it
      super
    end
  end
end

One important thing to remember when using method_missing that one should also override respond_to? method:

class Numeric
   def respond_to?(method_name, include_all = false) 
     method_name.to_s.match(/^is_greater_than_(\d+)\?$/) || super
   end
end

Forgetting to do so leads to a inconsistent situation, when you can successfully call 600.is_greater_than_123, but 600.respond_to(:is_greater_than_123) returns false.

Found a mistake? Have a question or improvement idea? Let me know.

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