unfold

Here's a little something from my old blog that I found myself using again. If you want the background, you might like my old posts Really simple anamorphisms in Ruby and Really useful anamorphisms in Ruby.

This week-end, I found myself thinking about #unfold in a new way: it's a way of turning an iteration into a collection. So anywhere you might use for or while or until, you can use #unfold to create an ordered collection instead. This allows you to use all of your existing collection methods like #select, #reject, #map, and of course #inject instead of writing out literal code to accomplish the same thing.

For example, given a class, here are all of the classes and superclasses that implement the class method #foo:

class Foo
  
  def self.foo_handlers
    self.unfold(&:superclass).select { |klass| klass.respond_to?(:foo) }
  end

end

The little idea here is that you don't have to write a loop of some sort climbing the class tree and accumulating an array of classes that respond to #foo as you go. This expresses the same idea in terms of operations on a collection.