From: Brian Candler <b.candler@...>
Date: 2008-12-03T06:11:01+09:00
Subject: Re: dynamic programming

Frank Tao wrote:
> # I have a class Adam
> # I want to modify the method("m_a") so that it will return the cached
> result
> # If no cached result is available, then return the original result
> # I want to create a class method(AKA: macro) to make it DRY

You might want to look at the 'memoize' method in ActiveRecord trunk.

> # So far, I have issues like
> # 1) dynmically define class variable

I suggest: don't use a class variable :-) An instance variable of the 
class would be fine. But personally I wouldn't keep the memoized values 
in the class; I'd keep them in the instances themselves.

> # 2) failed to include a module inside a method of a class

Look at Module.included for this, as shown below.

There are probably cleaner and/or more efficient ways than the 
following, but it demonstrates the principle.

  module Cache
    def self.included(base)
      base.extend ClassMethods
    end

    module ClassMethods
      def caching_method(m)
        name = "orig_#{m}"
        var = "@__#{m}"
        alias_method name, m
        define_method(m) { |*args|
          return instance_variable_get(var) if 
instance_variable_defined?(var)
          instance_variable_set(var, send(name,*args))
        }
      end
    end
  end

  class Adam
    include Cache
    def foo
      rand(100)
    end
    caching_method :foo
  end

  a = Adam.new
  p a.foo
  p a.foo
  p a.foo

Beware of what you really want here though. If foo takes arguments, do 
you want foo(1) and foo(2) to be able to return different values? Do you 
want them both to be cached? If so, I leave that as an exercise for you.
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