demonstrating the difference between method and constant lookup

method_vs_constant_lookup.rb

#!/usr/bin/env ruby

module TopLevel
  CONST_D = :d_from_top_level
  module MiddleLevel
    class BottomLevel
      CONST_A = :foo
      CONST_B = :bar
      CONST_C = :c_from_bottom_level

      # does not behave like you would expect for method lookup
      def const_c; CONST_C; end
      def const_d; CONST_D; end

      # just won't work
      def const_undefined; CONST_DEFINED_IN_CLASS_A; end

    end
  end
end

class A < TopLevel::MiddleLevel::BottomLevel
  CONST_B = "Abar"
  CONST_C = "c from class A"
  CONST_D = "d from class A"
  CONST_DEFINED_IN_CLASS_A = "only defined in class A"

  # behaves like you would expect method lookup:
  def const_a; CONST_A; end
  def const_b; CONST_B; end
  def our_own_const_d; CONST_D; end
  # might confuse you, if you aren't careful:
  def const_d_from_a; const_d; end

  # just to demonstrate the flipside of const_undefined
  def const_defined_here; CONST_DEFINED_IN_CLASS_A; end

end

def safely_puts
  puts yield
rescue => ex
  puts "Exception!  =>  #{ex}"
end

safely_puts { A.new.const_a }
safely_puts { A.new.const_b }
safely_puts { A.new.const_c }
safely_puts { A.new.const_d }
safely_puts { A.new.const_d_from_a }
safely_puts { A.new.our_own_const_d }
safely_puts { A.new.const_undefined }
safely_puts { A.new.const_defined_here }

results.txt

foo
Abar
c_from_bottom_level
d_from_top_level
d_from_top_level
d from class A
Exception!  =>  uninitialized constant TopLevel::MiddleLevel::BottomLevel::CONST_DEFINED_IN_CLASS_A
only defined in class A