constants defined in Kernel are also defined in Object?

Discussion in 'Ruby' started by Paul Brannan, Feb 28, 2008.

  1. Paul Brannan

    Paul Brannan Guest

    This seems peculiar to me:

    [email protected]:~/tmp$ ruby -e 'module M; FOO = 42; end; class Foo; include M; end; p Foo.const_defined?:)FOO)'
    [email protected]:~/tmp$ ruby -e 'Kernel.const_set:)FOO, 42); p Object.const_defined?:)FOO)'
    [email protected]:~/tmp$ ruby -e 'Kernel.const_set:)FOO, 42); p Class.const_defined?:)FOO)'

    Is there a reason for this behavior?

    Paul Brannan, Feb 28, 2008
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  2. % ruby -e 'p Object.ancestors'
    [Object, Kernel]

    Kernel is included by Object.
    Gregory Seidman, Feb 28, 2008
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  3. And M is included by Foo, yet the analogy breaks down in the outputs of


    Joel VanderWerf, Feb 28, 2008
  4. Hunh. I didn't read closely. A little playing in irb reveals the same
    thing. That's odd. I have no answer for you.
    Gregory Seidman, Feb 28, 2008
  5. Paul Brannan

    Jeremy Evans Guest

    Yes. Namespaces:

    irb(main):001:0> module M
    irb(main):002:1> FOO = 42
    irb(main):003:1> end
    => 42
    irb(main):004:0> class Foo
    irb(main):005:1> include M
    irb(main):006:1> end
    => Foo
    irb(main):007:0> Foo.const_defined?:)FOO) # looks for Foo:FOO
    => false
    irb(main):008:0> Foo::M.const_defined?:)FOO) # looks for Foo::M::FOO
    (irb):8: warning: toplevel constant M referenced by Foo::M
    => true
    irb(main):009:0> Object.const_defined?:)FOO) # looks for FOO
    => false
    irb(main):010:0> Kernel.const_set:)FOO, 42) # sets FOO
    => 42
    irb(main):011:0> Object.const_defined?:)FOO) # looks for FOO
    => true
    irb(main):012:0> Class.const_defined?:)FOO) # looks for Class::FOO
    => false
    irb(main):013:0> Class.const_set:)FOO, 42) # sets Class::FOO
    => 42
    irb(main):014:0> Class.const_defined?:)FOO) # looks for Class::FOO
    => true

    Jeremy Evans, Feb 28, 2008
  6. Paul Brannan

    Arlen Cuss Guest


    How about this?
    Arlen Cuss, Feb 28, 2008
  7. There still seems to be something unique in the relationship between
    Kernel and Object. Can you replicate it?

    module MyKernel
    FOO = 42

    module Kernel
    FOO = 42

    class MyObject
    include MyKernel
    extend MyKernel
    # what else do we have to do?

    # Alternatives to assigning to FOO above
    MyKernel.const_set:)BAR, 42)
    Kernel.const_set:)BAR, 42)

    p MyObject.const_defined?:)FOO) # ==> false
    p MyObject.const_defined?:)BAR) # ==> false

    p Object.const_defined?:)FOO) # ==> true
    p Object.const_defined?:)BAR) # ==> true

    Can you make the outputs be true in the MyObject case?

    The following works for constants that are already defined _only_ (which
    is _not_ how Object and Kernel behave):

    class MyObject
    MyKernel.constants.each do |c|
    const_set(c, MyKernel.const_get(c))
    Joel VanderWerf, Feb 28, 2008
  8. But:

    Kernel.const_set:)FOO, 42)
    class Foo; end
    p Foo::FOO # ==> 42
    p Foo.const_defined?:)FOO) # ==> false
    p Object.const_defined?:)FOO) # ==> true
    Joel VanderWerf, Feb 28, 2008
  9. Paul Brannan

    Arlen Cuss Guest

    Indeed! It works in both cases because Foo is an Object, Object
    includes Kernel -- Foo won't tell you that FOO is defined because Foo
    didn't define it itself!

    I've been writing a new parser and byte-code VM for Ruby recently
    (, excuse the advertising), which meant
    figuring out the whole Ruby object model. This had me tricked at
    several stages, but here's how I summarised it to myself:

    1. Foo is an instance of Object [eventually].
    2. Object mixes in Kernel.
    2.a. Kernel's constants are Object's too.
    3. Foo::SOME_CONST will see through to Object::SOME_CONST and then
    that to Kernel::SOME_CONST.

    Correct me if I'm wrong in some detail.

    Arlen Cuss, Feb 28, 2008
  10. Paul Brannan

    Jeremy Evans Guest

    I think that Kernel and Object are special in that their constants are
    available in the top level namespace. Other than there isn't much
    difference between them and other classes in this case.
    irb(main):001:0> module MyKernel
    irb(main):002:1> def self.included(klass)
    irb(main):003:2> klass.const_set:)FOO, 42)
    irb(main):004:2> end
    irb(main):005:1> end
    => nil
    irb(main):006:0> class MyObject
    irb(main):007:1> include MyKernel
    irb(main):008:1> end
    => MyObject
    irb(main):009:0> MyObject.const_defined?:)FOO)
    => true
    If I understand you correctly, you want all constants in MyKernel to be
    available in MyObject directly, even if the constants are added to
    MyKernel after after it is included in MyObject. If that is what you
    want, you'll probably have to use the MyKernel.included method to store
    a list of classes that include MyKernel, and override const_set on
    MyKernel to add the constants to those classes in addition to adding it
    to itself.

    Alternatively, if you want MyObject to have direct access to any
    constants in any included modules, you could override const_get on
    MyObject to check all included_modules if the constant isn't found in

    I'm not sure why you would want to do either of those things, though.

    Jeremy Evans, Feb 29, 2008
  11. Paul Brannan

    Arlen Cuss Guest

    [Note: parts of this message were removed to make it a legal post.]

    The reason for that is since the top level namespace *is* an Object.

    `main' is an instance of Object, hence everything we evaluate is just like
    it was evaluated in the instance method of any other Object. We have access
    to Object's constants, and Object includes Kernel, so Kernel's too.

    I'm not sure why you would want to do either of those things, though.
    Arlen Cuss, Feb 29, 2008
  12. Paul Brannan

    Arlen Cuss Guest

    [Note: parts of this message were removed to make it a legal post.]


    Just to clarify everything else, this is the reason why they're available on
    every object. `Functions' defined on the top-level are actually *instance
    methods of Kernel*. This means that they're *included* as instance methods
    of Object, and hence of *every* object. Since Kernel is itself an Object,
    and it has a metaclass which is by all means an Object, this means you can
    even access them on Kernel as if they were module methods. Or any other
    => nilhi!
    => nilhi!
    => nilhi!
    => nil
    Arlen Cuss, Feb 29, 2008
  13. Paul Brannan

    Arlen Cuss Guest

    [Note: parts of this message were removed to make it a legal post.]


    Aha! Actually, it's great to see you reply to this entry. I've seen you
    comment in this list in the past on this topic (173746, 207801) which was
    part of my attempt to understand it all.

    For example #inspect is defined in Kernel, and you can write
    I see. If I try the way you do, I get the same result. If anything, this
    tells me I should be careful about using irb to work this stuff out, as it
    produces a different (!) result;Object#inspect
    => "12"
    Guy Decoux
    Cheers! :)
    Arlen Cuss, Feb 29, 2008
  14. Paul Brannan

    Paul Brannan Guest

    Apparently all constants of modules mixed into Object belong to Object:

    irb(main):001:0> module M; FOO = 42; end
    => 42
    irb(main):002:0> class Object; include M; end
    => Object
    irb(main):003:0> class Foo; end
    => nil
    irb(main):004:0> Object.const_defined?:)M)
    => true
    irb(main):005:0> Foo.const_defined?:)M)
    => false

    The culprit:

    variable.c:1420 if (!recurse && klass != rb_cObject) break;
    Paul Brannan, Mar 4, 2008
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