Blocks In-depth: instance_eval

clean_room.rb

class MyLibrary
  class Configuration
    attr_accessor :protocol, :hostname, :port
  end

  attr_accessor :configuration

  def configure(&block)
    self.configuration ||= Configuration.new
    self.configuration.instance_eval(&block)
  end
end

m = MyLibrary.new

m.configure do |config|
  config.protocol = "https"
  config.hostname = "localhost"
  config.port = 80
end

clean_room_dsl.rb

class MyLibrary
  class Configuration
    def protocol(protocol)
      @protocol = protocol
    end

    def hostname(hostname)
      @hostname = hostname
    end

    def port(port)
      @port = port
    end
  end

  attr_accessor :configuration

  def configure(&block)
    self.configuration ||= Configuration.new
    self.configuration.instance_eval(&block)
  end
end

m = MyLibrary.new

m.configure do |config|
  protocol "https"
  hostname "localhost"
  port 80
end

instance_eval.rb

p = proc { puts to_a }

[1,2,3,4,5].instance_eval(&p) # => [1,2,3,4,5]
OpenStruct.new(to_a: "HAI").instance_eval(&p) # => HAI