AkitaOnRails

class Object
  def lisp_string
    to_s
  end
end

class NilClass
  def lisp_string
    "nil" 
  end
end

class Array
  # Convert an Array into an S-expression 
  # (i.e. linked list).
  # Subarrays are converted as well.
  def sexp
    result = nil
    reverse.each do |item|
      item = item.sexp if item.respond_to?(:sexp)
      result = cons(item, result)
    end
    result
  end
end

# The Basic Lisp Cons cell data structures.  
# Cons cells consist of a head and a tail.
class Cons
  attr_reader :head, :tail

  def initialize(head, tail)
    @head, @tail = head, tail
  end

  def (other)
    return false unless other.class  Cons
    return true if self.object_id  other.object_id
    return car(self)  car(other) && cdr(self) == cdr(other)
  end

  # Convert the lisp expression to a string.
  def lisp_string
    e = self
    result = “(” 
    while e
      if e.class != Cons
        result << ”. ” << e.lisp_string
        e = nil
      else
        result << car(e).lisp_string
        e = cdr(e)
        result << ” ” if e
      end
    end
    result << “)” 
    result
  end
end

# Lisp Primitive Functions.

# It is an atom if it is not a cons cell.
def atom?(a)
  a.class != Cons
end

# Get the head of a list.
def car(e)
  e.head
end

# Get the tail of a list.
def cdr(e)
  e.tail
end

# Construct a new list from a head and a tail.
def cons(h,t)
  Cons.new(h,t)
end

# Here is the guts of the Lisp interpreter. 
#  Apply and eval work together to interpret 
# the S-expression.  These definitions are taken
# directly from page 13 of the Lisp 1.5 
# Programmer’s Manual.

def apply(fn, x, a)
  if atom?(fn)
    case fn
    when :car then caar(x)
    when :cdr then cdar(x)
    when :cons then cons(car(x), cadr(x))
    when :atom then atom?(car(x))
    when :eq then car(x)  cadr(x)
    else
      apply(eval(fn,a), x, a)
    end
  elsif car(fn)  :lambda
    eval(caddr(fn), pairlis(cadr(fn), x, a))
  elsif car(fn)  :label
    apply(caddr(fn), x, 
      cons(cons(cadr(fn), caddr(fn)), a))
  end
end

def eval(e,a)
  if atom?(e)
    cdr(assoc(e,a))
  elsif atom?(car(e))
    if car(e)  :quote
      cadr(e)
    elsif car(e)  :cond
      evcon(cdr(e),a)
    else
      apply(car(e), evlis(cdr(e), a), a)
    end
  else
    apply(car(e), evlis(cdr(e), a), a)
  end
end

# And now some utility functions used 
# by apply and eval.  These are
# also given in the Lisp 1.5
# Programmer's Manual.

def evcon(c,a)
  if eval(caar(c), a)
    eval(cadar(c), a)
  else
    evcon(cdr(c), a)
  end
end

def evlis(m, a)
  if m.nil?
    nil
  else
    cons(eval(car(m),a), evlis(cdr(m), a))
  end
end

def assoc(a, e)
  if e.nil?
    fail "#{a.inspect} not bound" 
  elsif a  caar(e)
    car(e)
  else
    assoc(a, cdr(e))
  end
end

def pairlis(vars, vals, a)
  while vars && vals
    a = cons(cons(car(vars), car(vals)), a)
    vars = cdr(vars)
    vals = cdr(vals)
  end
  a
end

# Handy lisp utility functions built on car and cdr.

def caar(e)
  car(car(e))
end

def cadr(e)
  car(cdr(e))
end

def caddr(e)
  car(cdr(cdr(e)))
end

def cdar(e)
  cdr(car(e))
end

def cadar(e)
  car(cdr(car(e)))
end

require 'lisp'

# Create an environment where 
# the reverse, rev_shift and null
# functions are bound to an 
# appropriate identifier.

env = [
  cons(:rev_shift,
    [:lambda, [:list, :result],
      [:cond,
        [[:null, :list], :result],
        [:t, [:rev_shift, [:cdr, :list],
            [:cons, [:car, :list], :result]]]]].sexp),
  cons(:reverse,
    [:lambda, [:list], [:rev_shift, :list, nil]].sexp),
  cons(:null, [:lambda, [:e], [:eq, :e, nil]].sexp),
  cons(:t, true), 
  cons(nil, nil)
].sexp

# Evaluate an S-Expression and print the result

exp = [:reverse, [:quote, [:a, :b, :c, :d, :e]]].sexp

puts "EVAL: #{exp.lisp_string}" 
puts "  =>  #{eval(exp,env).lisp_string}"