Lazy evaluation in Ruby

Question

I have a situation for Ruby, where an object is possibly necessary to be created, but it is not sure. And as the creation of the object might be costly I am not too eager creati

Answer 1

If you want to lazily evaluate pieces of code, use a proxy:

class LazyProxy

  # blank slate... (use BasicObject in Ruby 1.9)
  instance_methods.each do |method| 
    undef_method(method) unless method =~ /^__/
  end

  def initialize(&lazy_proxy_block)
    @lazy_proxy_block = lazy_proxy_block
  end

  def method_missing(method, *args, &block)
    @lazy_proxy_obj ||= @lazy_proxy_block.call # evaluate the real receiver
    @lazy_proxy_obj.send(method, *args, &block) # delegate unknown methods to the real receiver
  end
end

You then use it like this:

expensive_object = LazyProxy.new { ExpensiveObject.new }
expensive_object.do_something

You can use this code to do arbitrarily complex initialization of expensive stuff:

expensive_object = LazyProxy.new do
  expensive_helper = ExpensiveHelper.new
  do_really_expensive_stuff_with(expensive_helper)
  ExpensiveObject.new(:using => expensive_helper)
end
expensive_object.do_something

How does it work? You instantiate a LazyProxy object that holds instructions on how to build some expensive object in a Proc. If you then call some method on the proxy object, it first instantiates the expensive object and then delegates the method call to it.