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].cdr   => [2,3] [2,3].cons(1) => [1,2,3] 

(in lisp):

(car '(1 2 3))  => 1 (cdr '(1 2 3))  => (2 3) (cons 1 '(2 3)) => (1 2 3) 

Answer 1

Ruby arrays are not implemented as singly-linked lists, so it is not as useful to have car and cdr and stuff.

If you really wanted, you could do

[1,2,3][0]      => 1 [1,2,3].first   => 1 [1,2,3][1..-1]  => [2,3] [1] + [2,3]     => [1,2,3] 

Answer 2

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))