Documentation

• 26

  JUN
  2008

  Summoning Error Classes As Needed

  In the past, I've written a lot of code like this:

  class SomeThing
    class SomeError       < RuntimeError; end
    class AnotherError    < RuntimeError; end
    class YetAnotherError < RuntimeError; end
  
    # some methods that can raise the above errors...
  end
  

  I have a new strategy I've been using for code like this and it has really been working out well. Here is how I do the same thing today:

  class SmarterThing
    def self.const_missing(error_name)  # :nodoc:
      if error_name.to_s =~ /Error\z/
        const_set(error_name, Class.new(RuntimeError))
      else
        super
      end
    end
  
    # error raising methods here...
  end
  

  Let's discuss how this works. The const_missing() method is a hook in Ruby, much like the beloved method_missing(). Note that const_missing() is a class method though, instead of an instance method. When a constant is used and Ruby can't find it, a call to the hook will be triggered.

  In this version, I just check to see if the constant name ends in Error. If it does, I summon an Exception subclass on the fly. Other calls to this hook are forwarded on to Ruby's default error raising implementation via super.

  Read more…

  In: Ruby Voodoo | Tags: Documentation & Error Handling | 5 Comments