Accessing a constant

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 #=> uninitial         


        

Answer 1

From the documentation of instance_eval (emphasis mine):

Evaluates a string containing Ruby source code, or the given block, within the context of the receiver (obj). In order to set the context, the variable self is set to obj while the code is executing, giving the code access to obj’s instance variables.

Nothing more is done here. In particular, the constant assignment runs in the enclosing context of the block. Observe:

irb(main):001:0> module A
irb(main):002:1>   module B; end
irb(main):003:1>   B.instance_eval { C = 1 }
irb(main):004:1> end
=> 1
irb(main):006:0> A::C
=> 1