def .. end is ok and class .. end is not ok .. Why??

[Kyung won Cheon]

module M
end

M.module_eval { def self.a; puts 'ok' end }
M.a
# => 'ok'

M.module_eval { class A; end }
M::A

# => uninitialized constant M::A (NameError)

 

[Pit Capitain]

module M
end

M.module_eval { def self.a; puts 'ok' end }
M.a
# => 'ok'

M.module_eval { class A; end }
M::A

# => uninitialized constant M::A (NameError)

Constants (including class and module names) are lexically scoped (at
least in Ruby 1.8). Your code therefore defines the top-level class A.
If you want a nested constant, you could do

M.module_eval { class self::A; end }

Regards,
Pit