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Reducing magic in Ruby’s Forwardable class implementation


Using string-eval in Ruby for metaprogramming is unnecessarily obscuring. Ruby’s more modern and specific metaprogramming methods should be used instead whenever possible. This problem is illustrated on the example of Ruby’s Forwardable class.

In detail…

Ruby’s Forwardable class is using metaprogramming to forward calls from a frontend interface to an instance in the back executing the call.

Metaprogramming is the discipline of making code that creates code. This task allready is rather abstract and hard to grasp in itself. Having hard to grasp code is a liability. One of the goals of writing code is allways to keep the code as simple and as well understandable as possible.

Additionaly, metaprogramming code itself is difficult to read and understand: that is because the metaprogramming code will not necessarily express what the code it is creating is about, but only how it is creating that code. As such the code it is creating can be invisible to you as a reader of the source code - the created code will only start to exist at runtime.

One would therefore expect that programmers would try especially hard when they metaprogram to make that particular kind of code expressive and easy to understand.

Another consequence of the fact that the code produced by metaprogramming is not necessarily visible, is that debugging becomes more difficult: when analyzing problems you’ll not only be unsure how the programm works, but in addition, you won’t even be sure how the code that is executed looks like - since it is only generated at runtime.

This post is focusing on the last problem: debugging of metaprogrammed code.

There are two approaches to metaprogramming. One is to have as far as possible compile-time parseable code and the other is to let the code only be parsed at runtime.

As of version 1.9.2, Ruby’s Forwardable class is using the latter. The metaprogramming code in Ruby 1.8.7 looks like this:

    module_eval(<<-EOS, "(__FORWARDABLE__)", 1)
      def #{ali}(*args, &block)
          #{accessor}.__send__(:#{method}, *args, &block)
        rescue Exception
          $@.delete_if{|s| /^\\(__FORWARDABLE__\\):/ =~ s} unless Forwardable::debug

As said, this has the consequence of the metaprogrammed code being completely invisible to the parser and other tools such as editors and debuggers.

This results in the following:

$ cat queue.rb
require 'rubygems'
require 'forwardable'
require 'ruby-debug'

class Queue
  extend Forwardable

  def initialize
    @q = [ ]    # prepare delegate object

  # setup preferred interface, enq() and deq()...
  def_delegator :@q, :push, :enq
  def_delegator :@q, :shift, :deq

  # support some general Array methods that fit Queues well
  def_delegators :@q, :clear, :first, :push, :shift, :size

q = Queue.new
debugger # ------ DEBUGGING FROM HERE ON -----
q.enq 1, 2, 3, 4, 5
q.push 6

q.shift    # => 1
while q.size > 0
  puts q.deq

q.enq "Ruby", "Perl", "Python"
puts q.first
puts q.first

$ ruby queue.rb

q.enq 1, 2, 3, 4, 5

(rdb:1) step

(rdb:1) list =
*** No sourcefile available for (__FORWARDABLE__)

(rdb:1) step

In other words, you are rather lost allready - otherwise you probably wouldn’t be stepping through your code - and in that situation it happens that your debugger gets completely lost as well, since it does not know any more where in the code it is and what it exactly is executing.

That’s nothing the programmer wishes for. In a situation where you are debugging you want to have a maximally clear view of all state, including what code you are currently executing.

Chaning that situation requires making as much of the metaprogrammed code visible to the parser, which is the second approach to metaprogramming mentioned previously:

$ cat forwardable2.rb
    self.send(:define_method, ali) do |*args,&block|
        instance_variable_get(accessor).__send__(method, *args,&block)
      rescue Exception
        $@.delete_if{|s| /^\\(__FORWARDABLE__\\):/ =~ s} unless Forwardable2::debug

Note that it’s the same code as before, except that we do not do eval("string") any more, but instead are using specific, more modern metaprogramming tools provided by standard Ruby.

The result is the following:

$ ruby queue.rb

q.enq 1, 2, 3, 4, 5

(rdb:1) step

(rdb:1) list =
[144, 153] in /usr/lib/ruby/1.8/forwardable2.rb
   144      accessor = accessor.id2name if accessor.kind_of?(Integer)
   145      method = method.id2name if method.kind_of?(Integer)
   146      ali = ali.id2name if ali.kind_of?(Integer)
   148      self.send(:define_method, ali) do |*args,&block|
=> 149        begin
   150          instance_variable_get(accessor).__send__(method, *args,&block)
   151        rescue Exception
   152          $@.delete_if{|s| /^\\(__FORWARDABLE__\\):/ =~ s} unless Forwardable2::debug
   153          Kernel::raise

(rdb:1) step
instance_variable_get(accessor).__send__(method, *args,&block)

Allready much, much better.

Of course, with the string-eval approach to metaprogramming Ruby itself could do better by saving the string that is being evaled to be able to refer to it later at step-through time. However currently we don’t have this option.

Tomáš Pospíšek