a demonstration of the ||= idiom

orequals.rb

#!/usr/bin/env ruby -wKU
#
# a demonstration of the ||= idiom
#
#   x ||= y
#
# is usually translated as
#
#   x = x || y
#
# but as demonstrated here, is better translated as
#
#   x || x = y
#

class A
  def initialize
    @x = nil
  end

  def x
    puts "  returning x: #{@x.inspect}"
    @x
  end

  def x=(value)
    puts "  setting x to #{value.inspect}"
    @x = value
  end
end

y = "y"

puts "\n----- retrieval -----"
a = A.new
a.x

puts "\n----- assignment -----"
a = A.new
a.x = "foo"

puts "\n----- a.x = a.x || y -----"
a = A.new
a.x = a.x || y
puts "and again:"
a.x = a.x || y

puts "\n----- a.x || a.x = y -----"
a = A.new
a.x || a.x = y
puts "and again:"
a.x || a.x = y

puts "\n----- a.x ||= y -----"
a = A.new
a.x ||= y
puts "and again:"
a.x ||= y

puts <<-summary

----- and thus -----

  x ||= y

only assigns y if x isn't already set, i.e.

  x || x = y

and not

  x = x || y

summary

# see also: http://www.ruby-forum.com/topic/151660/