From: Greg Weeks <greg.weeks@...>
Date: 2007-11-14T15:23:35+09:00
Subject: "once" meta-method without "module_eval" (Pickaxe)

My impression is that Ruby meta-programming used to lean heavily on
evaluating strings as code, but is moving away from that.  (Good!)  In
that spirit, it would be nice to replace this example from Chapter 24,
Classes and Objects:

class Date
  class <<self
    def once(*ids) # :nodoc:
      for id in ids
        module_eval <<-"end;"
          alias_method :__#{id.to_i}__, :#{id.to_s}
          private :__#{id.to_i}__
          def #{id.to_s}(*args, &block)
            (@__#{id.to_i}__ ||= [__#{id.to_i}__(*args, &block)])[0]
          end
        end;
      end
    end
  end
  ...
  once :as_string, :as_YMD
end

"once" replaces a specified method with a method that caches the return
value on the first call.

The example above served to illustrate the use of class instance
variables.  However, the replacement that comes to mind does not:

  class <<self
    def once(*ids)
      values = {}
      for id in ids
        new_id = "__#{id.to_i}__".to_sym
        alias_method new_id, id
        private new_id
        define_method (id) do |*args|
          (values[id] ||= [ method(new_id).call(*args) ])[0]
        end
      end
    end
  end

Is there some way to avoid evals and still use class instance variables?

Is there some way to avoid evals and still handle the "&block" argument?

Is there a natural alternative to the local hash variable?

Am I right to think that eval-ing strings is retro?

(By the way, neither implementation allows "once" to be called twice
with the same symbol.)
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