Why in Ruby do uninitialized instance variables return nil, but uninitialized class variables raise an error?

Question

In Ruby, why does an uninitialized instance variable return nil while an uninitialized class variable raises a NameError?

Compare:

@some_uninitialized_variable # => nil

and:

@@some_uninitialized_class_variable # => NameError

Answer 1

My take is the following:

• uninitialized local variables return a name error because Ruby doesn't know if its intended to be a local variable or a non-existent method.

• if uninitialized class variables returned nil when not defined, it could lead to nasty bugs when the variable was actually assigned the value nil by a distant ancestor. That is, I see this as protecting the coder.

• having instance variables default to nil when uninitialized if an oft-used feature: @a = @a || [].