lisp.rb

# Syntax of Ruby is flexible enough to allow writing Lisp-like code.

# We need few helper functions (no Lisp yet):

def list(*args)
  args
end

def car(list)
  list.first
end

def cdr(list)
  list[1..-1]
end

def print(args)
  p args
end

def define(function, body)
  name = function.first
  args = function[1..-1].map { |a| a.to_s }.join(", ")

  eval <<-EOS
    def #{name}(#{args})
      #{body}
    end
  EOS
end

# And now Lisp!

(print (car (list 1, 2, 3)))       # => 1
(print (cdr (list 1, 2, 3)))       # => [2, 3]
(print (car (cdr (list 1, 2, 3)))) # => 2

(define [:square, :x], "x*x")
(print (square 3))                 # => 9

# The same in Scheme:
#
# (print (car (list 1 2 3)))
# (print (cdr (list 1 2 3)))
# (print (car (cdr (list 1 2 3))))
#
# (define (square x) (* x x))
# (print (square 3))