as.rb

module Bar
  def do_sth
    puts "@proxy: #{@proxy.inspect}"
  end
end

class Foo
  include Bar

  def initialize
    @proxy = "john"
  end

  def as(proxy, &block)
    puts self.inspect

    # ----------------
    # works, yet cumbersome and not thread-safe:
    bak = @proxy
    # this value should trump the block's binding value for @a
    @proxy = proxy
    block.call
    @proxy = bak
    
    # ----------------
    # wip (fails):
    o = self.class.new
    o.instance_variable_set("@proxy", proxy)
    o.instance_eval do
      puts self.inspect
      puts @proxy.inspect
      block.call
    end
  end
end

Foo.new.instance_eval do
  do_sth # => should output @proxy: john
  puts
  as('root') { do_sth } # => should output @proxy: root 
  puts
  do_sth # => should output @proxy: john
end