Ruby Language Metaprogramming Defining methods dynamically


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
      # if the method_name doesn't match what we want, let the previous definition of `method_missing` handle it

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

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.