更详细地说,我有一个模块Narf,它为一系列类提供了基本功能.具体来说,我想影响继承Enumerable的所有类.所以我把Narf包含在Enumerable中.
Array是一个默认包含Enumerable的类.然而,它并没有受到Narf在模块中的后期加入的影响.
有趣的是,在包含之后定义的类从Enumerable获得Narf.
例：
# This module provides essential features
module Narf
def narf?
puts "(from #{self.class}) ZORT!"
end
end
# I want all Enumerables to be able to Narf
module Enumerable
include Narf
end
# Fjord is an Enumerable defined *after* including Narf in Enumerable
class Fjord
include Enumerable
end
p Enumerable.ancestors # Notice that Narf *is* there
p Fjord.ancestors # Notice that Narf *is* here too
p Array.ancestors # But,grr,not here
# => [Enumerable,Narf]
# => [Fjord,Enumerable,Narf,Object,Kernel]
# => [Array,Kernel]
Fjord.new.narf? # And this will print fine
Array.new.narf? # And this one will raise
# => (from Fjord) ZORT!
# => NoMethodError: undefined method `narf?' for []:Array