Question

I have recently seen some code that I do not completely understand. There is an array named foo that contains instances of Proc objects. Then, an env object is used to setup an environment to work in:

env = Object.new
foo.each do |f|
    env.instance_eval &f # what happens here?
end

What exactly happens when you open the object with instance_eval and pass &f as an argument? What happens to env at this point and the Proc itself?

Answer 1

The Proc gets executed in the context of env. It is as if you are calling a method on env: the block has access to its instance variables and public and private methods.

env = Object.new

env.instance_variable_set :@test, "test"

class << env
  private
  def test
    @test
  end
end

env.instance_eval { @test } #=> "test"
env.instance_eval { test }  #=> "test"