poormanspatternmatching.rb

class Array
  def extract_options!
    last.is_a?(::Hash) ? pop : {}
  end

  def tail
    head, *tail = *self
    tail
  end

  def head
    head, *tail = *self
    head
  end

  def shape
    head, *tail = *self
    case 
    when (head.nil? and tail.empty?)
      :default
    when tail.empty?
      :head
    else
      :head_tail
    end
  end

  def match(*args)
    rules = args.extract_options!
    if rules.has_key?(*args)
      rules[*args].call 
    else 
      rules[:default].call(*args)
    end
  end


  # About 30 times slower than map though :/
  def pmap &block
    if size > 0 
      match self.shape,
        head_tail: lambda{ self.tail.pmap(&block).unshift yield(self.head) },
        head:      lambda{ [ yield(self.head) ]                            }
    else
      self
    end
  end
end