Ruby: how does constant-lookup work in instance_eval/class_eval?

Question

I'm working my way through Pickaxe 1.9, and I'm a bit confused by constant-lookup in instance/class_eval blocks. I'm using 1.9.2.

It seems that Ruby handles constant-lookup in *_eval blocks the same way it does method-lookup:

1. look for a definition in receiver.singleton_class (plus mixins);
2. then in receiver.singleton_class.superclass (plus mixins);
3. then continue up the eigenchain until you get to #<Class:BasicObject>;
4. whose superclass is Class;
5. and then up the rest of the ancestor chain (including Object, which stores all the constants you define at the top-level), checking for mixins along the way

Is this correct? The Pickaxe discussion is a bit terse.

Some examples:

class Foo
  CONST = 'Foo::CONST'
  class << self
    CONST = 'EigenFoo::CONST'
  end
end

Foo.instance_eval { CONST } # => 'EigenFoo::CONST'
Foo.class_eval { CONST } # => 'EigenFoo::CONST', not 'Foo::CONST'!
Foo.new.instance_eval { CONST } # => 'Foo::CONST'

In the class_eval example, Foo-the-class isn't a stop along Foo-the-object's ancestor chain!

And an example with mixins:

module M
  CONST = "M::CONST"
end
module N
  CONST = "N::CONST"
end

class A
  include M
  extend N
end

A.instance_eval { CONST } # => "N::CONST", because N is mixed into A's eigenclass
A.class_eval { CONST } # => "N::CONST", ditto
A.new.instance_eval { CONST } # => "M::CONST", because A.new.class, A, mixes in M

Answer 1

In 1.9.2 the constant lookup has changed again to be equivalent to the 1.8.7 behavior.

class A
  class B
    class C
    end
  end
end

A.class_eval { B } # => NameError
A.instance_eval { B } # => NameError
A.new.instance_eval { B } # => A::B

Basically, the constants are quasi-lexically scoped. This USED to be different between 1.9.x and 1.8.x branches, and it made library cross compatibility a pain, so they changed it back.

Yehuda Katz's (successful) appeal to restore 1.8 behavior

Answer 2

Constants are effectively lexically scoped so you cannot access them short hand outside of the module hierarchy within which they are defined. There is a good explanation here and slightly off topic but a good read here.