Question

Why can’t I access ‘B’ in the following from ‘A’ but can from the main environment?

module A; end
A.instance_eval{B=1}

B #=> 1
A::B #=> uninitialized

Answer 1

The idiomatic way to do this would be

 A.const_set(:B, 1)
 A::B #=> 1

As to why it doesn’t work, in Ruby 1.8 and 1.9.2+ (it was different in 1.9.1), constant lookup is lexically scoped. I found a good blog post with an explanation. To quote:

Note that these rules apply to constant definition as well as lookup.
In 1.8 and 1.9.2, a constant defined in a class_evaluated block will
be defined in the enclosing lexical scope, rather than the scope of
the receiver.

The same is also true for instance_eval.