trait.rb

# @see https://ruby-naseby.blogspot.com/2008/11/traits-in-ruby.html
module Trait
 class RequiredMethodMissing < RuntimeError; end

 def append_features( mod )
   unless mod.is_a? Class #not the best check, probably...
     list_of_methods = ( [ mod.get_required_methods ] << @required_methods ).flatten
     mod.required_methods( *list_of_methods.compact )
   else
     @required_methods.each do | id |
       unless mod.instance_methods( true ).include?( id.to_s )
         raise Trait::RequiredMethodMissing, id.to_s
       end
     end
   end
   super_methods = mod.instance_methods( true ) - mod.instance_methods( false )
   self.instance_methods( false ).each do | meth |
     if super_methods.include?( meth )
       alt_name = "_trait_#{self.class}_#{meth}"
       send( :alias_method, alt_name, meth )
       mod.module_eval <<-END_EVAL
       def #{meth}( *args, &block )
         #{alt_name}( *args, &block )
       end
       END_EVAL
     end
   end
   super
 end

 def extend_object( obj )
   append_features( obj.class )
 end

 def required_methods( *ids )
   @required_methods = ids
 end

 def get_required_methods; @required_methods; end
end

#tests
if $0 == __FILE__

 module T1
   extend Trait
   required_methods :to_s, :blah
   def bb; "T1#bb"; end
 end

 module T2
   extend Trait
   required_methods :t2hook
   include T1
   def cc; "T2#cc"; end
 end

 class A
   def to_s; end
   def blah; end
   def bb; "A#bb"; end
 end

 class B < A
 end

 class C; end

 class D; def t2hook; end; end;

 require 'test/unit'
 class TC_Trait < Test::Unit::TestCase
   def test_requirements_met
     a = A.new
     assert_nothing_raised { a.extend( T1 ) }
   end
  
   def test_requirements_not_met
     assert_raises( Trait::RequiredMethodMissing ) { C.new.extend( T1 ) }
   end
  
   def test_existing_method_not_overridden
     a = A.new
     a.extend T1
     assert_equal "A#bb", a.bb
   end
  
   def test_super_method_overridden
     b = B.new
     b.extend T1
     assert_equal "T1#bb", b.bb
   end
  
   def test_composed_trait_requirements_not_met
     assert_raises( Trait::RequiredMethodMissing ) { D.new.extend( T2 ) }
   end
  
   def test_composed_trait_requirements_met
     a = A.new
     a.class.send( :define_method, :t2hook, proc{} )
     assert_nothing_raised{ a.extend( T2 ) }
   end
 end

end