Reason #175 • June 24th, 2026

Conditional method definitions

In Ruby, we can define methods conditionally based on runtime conditions. This is useful for things like:

Ruby
class MyClass
  # Define methods based on Ruby version
  if RUBY_VERSION >= "3.0"
    def my_method
      puts "Running on Ruby 3.0 or later"
    end
  else
    def my_method
      puts "Running on an older version of Ruby"
    end
  end

  # Define methods based on the presence of a gem
  if defined?(SomeGem)
    def another_method
      puts "SomeGem is defined"
    end
  else
    def another_method
      puts "SomeGem is not defined"
    end
  end

  # Define methods based on environment variables
  if ENV["MY_ENV_VAR"] == "true"
    def env_method
      puts "Environment variable is set to true"
    end
  else
    def env_method
      puts "Environment variable is not set to true"
    end
  end
end

We could of course also do these checks inside of the methods, but defining them conditionally is more efficient, as it avoids unnecessary checks on every method call.

This touches on one of the most magical things about Ruby to me - that it's just Ruby all the way down, whether we're inside a class definition or inside a method. No special syntax or template language required

History

Conditional method definitions have been possible since Ruby's inception, because if, class, module, and def have always belonged to the same executable language rather than to a separate declaration system.

Reason #176 ?