modules including other modules

Discussion in 'Ruby' started by Tim Olsen, May 29, 2007.

  1. Tim Olsen

    Tim Olsen Guest

    Hello,

    It appears that including module A inside another module B can cause
    problems for classes that have already included A. What seems to
    happen is that a class can only see the methods from module B if that
    class include module A before module A includes module B.

    For example,

    module A

    def a
    puts "a"
    end

    end

    module B

    def b
    puts "b"
    end

    end

    class C

    include B

    def c
    puts "c"
    end

    end

    module B

    include A

    def b2
    puts "b2"
    end
    end


    C.new.c
    C.new.b
    C.new.b2
    C.new.a

    produces

    tolsen@neurofunk:~/svn/limeade$ /var/tmp/tst.rb
    c
    b
    b2
    /var/tmp/tst.rb:42: undefined method `a' for #<C:0x2ac204fe7180> (NoMethodError)

    Notice I can define b2() after C has already included B but including
    A into B does nothing for C.

    Any idea what's going on here?

    Thanks,
    Tim
     
    Tim Olsen, May 29, 2007
    #1
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  2. Tim Olsen

    Jeremy Henty Guest

    It's a known problem that is apparently almost impossible to fix. See
    http://eigenclass.org/hiki/The+double+inclusion+problem .

    Unless there's been some development that I've failed to Google up,
    your only option is "Don't Do That!". :-( Unless Ruby 1.9 handles
    things better.

    Regards,

    Jeremy Henty
     
    Jeremy Henty, May 29, 2007
    #2
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  3. Tim Olsen

    Trans Guest

    Also know as the "Dynamic Inclusion Problem" or just the "Inclusion
    Problem".
    While, it would be great if someone did find a way, from what I
    understand its too difficult to fix given how Ruby works, so don't
    hold your breath.

    T.
     
    Trans, May 29, 2007
    #3
  4. Adriano Mitre, Jan 6, 2011
    #4
  5. Update: the aforementioned solution was encapsulated into a gem which
    makes it feels like it were part of the Ruby language.

    Synopsis:
    Enumerable.module_eval { retroactively_include MyModule }

    At http://rubygems.org/gems/retroactive_module_inclusion one may find
    the link to the GitHub repository, documentation, bug tracker, etc.
     
    Adriano Mitre, Jan 20, 2011
    #5
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