Null Objects

Tom Locke and I have similar proclivities. He says:


class Object
  def _?()
    self
  end
end

class NilClass
  def _?()
    SafeNil.new
  end
end

class SafeNil
  def method_missing(*args, &b)
    nil.send(*args, &b) rescue nil
  end
end

I say:


class Object
  def null
    n = nil
    def n.method_missing(*args)
      null
    end
    n
  end
end

The difference:


string_or_nil._?.length        # Tom's
(string_or_nil||null).length   # mine

I like being able to go more than one method deep, 


(string_or_nil||null).length * 4

But that might not be to your taste.

August 25, 2007 | Filed Under Ruby

 

Comments

4 Responses to “Null Objects”

  1. Tom Locke on August 25th, 2007 10:31 am

    Nice. Is your line 4 supposed to read:

    def n.method_missing(*args)

    ?

  2. Daniel on August 25th, 2007 10:42 am

    Yes thanks, fixed now. Also instead of Object.new the singleton object needed to be nil, to evaluate as false. Should have checked before typing.

  3. Tyler Rick on October 12th, 2007 1:27 am

    I like how simple your implementation is compared to Tom’s.

    Unfortunately, I believe it has a fatal flaw: As soon as you call null once, the NilClass is permanently changed and *all* nil objects will have this new “safe nil” behavior.

    puts nil.foo rescue p $!
    # => #

    null

    puts nil.foo rescue p $!
    # => nil

    I don’t really understand why it has to be this way, but I’m guessing it has to do with the fact that nil itself is a “singleton object” (according to http://corelib.rubyonrails.org/classes/NilClass.html)

  4. Daniel on October 23rd, 2007 3:38 am

    Ah. I was wondering how long it would take you to work that out. :)
    (http://www.imdb.com/title/tt0062552/quotes)

    Let this be a lesson in how to shoot yourself in the foot with open core classes. Maybe I should be using Java so I don’t hurt myself.

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    Daniel Lucraft, London, UK. Interested in Ruby, Prolog and software.