Question

For example we have:

module X
 def test
  "X"
 end
end

module Y
 def test
  "Y"
 end
end

class L
 include X
end

L.new.test #=> "X"

class L
 include Y
end

L.new.test #=> "Y"

class L
 include X
end

L.new.test #=> "Y"

Here’s the strange thing. Why the last include didn’t change the method to X’s method?

Answer 1

Ruby does not allow to include one module twice, attempt to do it second time is just ignored.

From documentation of Module#append_features which is used by Module#include under the covers:

Ruby’s default implementation is to add the constants, methods, and module variables of this module to mod if this module has not already been added to mod or one of its ancestors.