Question

I’ve multiply heard Ruby touted for its super spectacular meta-programming capabilities, and I was wondering if anyone could help me get started with this problem.

I have a class that works as an “archive” of sorts, with internal methods that process and output data based on an input. However, the items in the archive in the class itself are represented and processed with integers, for performance purposes. The actual items outside of the archive are known by their string representation, which is simply number_representation.to_s(36).

Because of this, I have hooked up each internal method with a “proxy method” that converts the input into the integer form that the archive recognizes, runs the internal method, and converts the output (either a single other item, or a collection of them) back into strings.

The naming convention is this: internal methods are represented by _method_name; their corresponding proxy method is represented by method_name, with no leading underscore.

For example:

class Archive

  ## PROXY METHODS ##
  ## input: string representation of id's
  ## output: string representation of id's

  def do_something_with id
    result = _do_something_with id.to_i(36)
    return nil if result == nil
    return result.to_s(36)
  end

  def do_something_with_pair id_1,id_2
    result = _do_something_with_pair id_1.to_i(36), id_2.to_i(36)
    return nil if result == nil
    return result.to_s(36)
  end

  def do_something_with_these ids
    result = _do_something_with_these ids.map { |n| n.to_i(36) }
    return nil if result == nil
    return result.to_s(36)
  end

  def get_many_from id
    result = _get_many_from id
    return nil if result == nil         # no sparse arrays returned
    return result.map { |n| n.to_s(36) }
  end

  ## INTERNAL METHODS ##
  ## input: integer representation of id's
  ## output: integer representation of id's

  private

  def _do_something_with id
    # does something with one integer-represented id,
    # returning an id represented as an integer
  end

  def do_something_with_pair id_1,id_2
    # does something with two integer-represented id's,
    # returning an id represented as an integer
  end

  def _do_something_with_these ids
    # does something with multiple integer ids,
    # returning an id represented as an integer
  end

  def _get_many_from id
    # does something with one integer-represented id,
    # returns a collection of id's represented as integers
  end
end

There are a couple of reasons why I can’t just convert them if id.class == String at the beginning of the internal methods:

1. These internal methods are somewhat computationally-intensive recursive functions, and I don’t want the overhead of checking multiple times at every step
2. There is no way, without adding an extra parameter, to tell whether or not to re-convert at the end
3. I want to think of this as an exercise in understanding ruby meta-programming

Does anyone have any ideas?

---

edit

The solution I’d like would preferably be able to take an array of method names

@@PROXY_METHODS = [:do_something_with, :do_something_with_pair,
                   :do_something_with_these, :get_many_from]

iterate through them, and in each iteration, put out the proxy method. I’m not sure what would be done with the arguments, but is there a way to test for arguments of a method? If not, then simple duck typing/analogous concept would do as well.

---

I’ve come up with my own solution, using #class_eval

@@PROXY_METHODS.each do |proxy|
  class_eval %{ def #{proxy} *args
                  args.map! do |a|
                    if a.class == String
                      a.to_i(36)
                    else
                      a.map { |id| id.to_i(36) }
                    end
                  end
                  result = _#{proxy}(*args)

                  result and if result.respond_to?(:each)
                               result.map { |r| r.to_s(36) }
                             else
                               result.to_s(36)
                             end
                end
              }
end

However, #class_eval seems a bit…messy? or inelegant compared to what it “should” be.

Answer 1

class Archive
  # define a new method-creating method for Archive by opening the
  # singleton class for Archive
  class << Archive
    private # (make it private so no one can call Archive.def_api_method)
    def def_api_method name, &defn
      define_method(name) do |*args|
        # map the arguments to their integer equivalents,
        # and pass them to the method definition
        res = defn[ *args.map { |a| a.to_i(36) } ]
        # if we got back a non-nil response, 
        res and if res.respond_to?(:each)
                  # map all of the results if many returned
                  res.map { |r| r.to_s(36) } 
                else
                  # map the only result if only one returned
                  res.to_s(36)
                end
      end
    end
  end
  def_api_method("do_something_with"){ |id| _do_something_with(id) }
  def_api_method("do_something_with_pair"){ |id_1, id_2| _do_something_with_pair id_1.to_i(36), id_2.to_i(36) }
  #...
end

Instead of opening the singleton to define Archive.def_api_method, you could define it simply using

class Archive
  def Archive.def_api_method
    #...

But the reason I didn’t do that is then anyone with access to the Archive class could invoke it using Archive.def_api_method. Opening up the singleton class allowed me to mark def_api_method as private, so it can only be invoked when self == Archive.

If you’re always going to be calling an internal version with the same (or derivable) name, then you could just invoke it directly (rather than pass a definition block) using #send.

class Archive
  # define a method-creating method that wraps an internal method for external use
  class << Archive
    private # (make it private so no one can call Archive.api_method)
    def api_method private_name
      public_name = private_name.to_s.sub(/^_/,'').to_sym
      define_method(public_name) do |*args|
        # map the arguments to their integer equivalents,
        # and pass them to the private method
        res = self.send(private_name, *args.map { |a| a.to_i(36) })
        # if we got back a non-nil response, 
        res and if res.respond_to?(:each)
                  # map all of the results if many returned
                  res.map { |r| r.to_s(36) } 
                else
                  # map the only result if only one returned
                  res.to_s(36)
                end          end
      # make sure the public method is publicly available
      public public_name
    end
  end

  api_method :_do_something_with
  api_method :_do_something_with_pair

  private

  def _do_something_with
    #...
  end
  def _do_something_with_pair
    #...
  end
end

This is more like what is done by other meta-methods like attr_reader and attr_writer.