Ruby: how does constant-lookup work in instance_eval/class_eval?
- by Alan O'Donnell
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:
look for a definition in receiver.singleton_class (plus mixins);
then in receiver.singleton_class.superclass (plus mixins);
then continue up the eigenchain until you get to #<Class:BasicObject>;
whose superclass is Class;
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
