Lisp as an internal Ruby DSL?

Question

I've been able to find:

a) Lisp interpreters written Ruby (i.e., an external DSL)

http://onestepback.org/index.cgi/Tech/Ruby/LispInRuby.red

b) Prolog as a Ruby DSL

http://www.kdedevelopers.org/node/2369

c) Discussion of Ruby "as" a Lisp

http://www.randomhacks.net/articles/2005/12/03/why-ruby-is-an-acceptable-lisp

But oddly, I can't actually find an -internal- implementation of Lisp, like the one for Prolog. Am I just insufficiently Googly, or has nobody yet posted such a think?

Or is it possibly one can't quite do this in Ruby?

Answer 1

Here's the Ruby source code for the Lisp interpreter from page 13 of the Lisp Programmers manual:

  # Kernel Extensions to support Lisp
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

So let's say you've got the following Lisp code:

(defun reverse (list)
  (rev-shift list nil))

(defun rev-shift (list result)
  (cond ((null list) result)
    (t (rev-shift (cdr list) (cons (car list) result))) ))

You could render this in the DSL as:

  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}" 

(Original source for interpreter and examples can be found here.)

Update: Just realized you mentioned this solution in your question.