Are there ruby equivalents to car, cdr, and cons?

Question

Are there ruby equivalents to the lisp car, cdr, and cons functions? For those unfamiliar with lisp, here\'s what I want from ruby:

[1,2,3].car   => 1
[1,2,3]         


        

Answer 1

This is how you'd implement lisp-like single-linked lists in ruby:

class Object
  def list?
    false
  end
end

class LispNilClass
  include Enumerable
  def each
  end

  def inspect
    "lnil"
  end

  def cons(car)
    Cell.new(car, self)
  end

  def list?
    true
  end
end

LispNil = LispNilClass.new

class LispNilClass
  private :initialize
end

class Cell
  include Enumerable

  attr_accessor :car, :cdr

  def initialize(car, cdr)
    @car = car
    @cdr = cdr
  end

  def self.list(*elements)
    if elements.empty?
      LispNil
    else
      first, *rest = elements
      Cell.new(first, list(*rest))
    end
  end

  def cons(new_car)
    Cell.new(new_car, self)
  end

  def list?
    cdr.list?
  end

  # Do not use this (or any Enumerable methods) on Cells that aren't lists
  def each
    yield car
    cdr.each {|e| yield e}
  end

  def inspect
    if list?
      "(#{ to_a.join(", ") })"
    else
      "(#{car} . #{cdr})"
    end
  end
end

list = Cell.list(1, 2, 3) #=> (1, 2, 3)
list.list? #=> true
list.car #=> 1
list.cdr #=> (2, 3)
list.cdr.cdr.cdr #=> lnil
list.cons(4) #=> (4, 1, 2, 3)

notlist = Cell.new(1,2) #=> (1 . 2)
notlist.list? #=> false
notlist.car #=> 1
notlist.cdr #=> 2
notlist.cons(3) #=> (3 . (1 . 2))