;;;; -*- mode:Lisp; syntax:Common-Lisp; package:user -*- ;;;;
;;;; Copyright 1992 Patrick H. Winston.  All rights reserved.
;;;; Version 1.1, copied from master file on 21 Jan 93       
;;;; 
;;;; This software is licensed by Patrick H. Winston (licensor) for
;;;; instructional use with the textbooks ``Artificial Intelligence,'' by
;;;; Patrick H. Winston, and ``Lisp,'' by Patrick H. Winston and Berthold
;;;; K. P. Horn.  Your are free to make copies of this software and
;;;; modify it for such instructional use as long as:
;;;; 1. You keep this notice intact.
;;;; 2. You cause any modified files to carry a prominent notice stating
;;;;    that you modified the files and the date of your modifications.
;;;; This software is licensed ``AS IS'' without warranty and the licensor
;;;; shall have no liability for any alleged defect or damages.

;;;; REMARKS

#|

This version uses delayed evaluation.  Note that ENCAPSULATE
is designed so that each stream element is computed only once.

|#

;;;; BASIC ACCESS FUNCTIONS WITH DELAYED EVALUATION

(defun make-empty-stream () 'empty-stream)

(defun stream-endp (stream) (eq stream 'empty-stream))

(defun stream-first (stream) (first stream))

(defun stream-rest (stream)		
  (expose (second stream)))

(defmacro stream-cons (object stream)	
  `(list ,object (encapsulate ,stream)))

(defun stream-append (stream1 stream2)
  (if (stream-endp stream1)
      stream2
      (stream-cons (stream-first stream1)
                   (stream-append (stream-rest stream1)
                                  stream2))))

(defun stream-concatenate (streams)
  (if (stream-endp streams)
      'empty-stream
    (if (stream-endp (stream-first streams))
	(stream-concatenate (stream-rest streams))
      (stream-cons (stream-first (stream-first streams))
		   (stream-concatenate
		     (stream-cons (stream-rest (stream-first streams))
				  (stream-rest streams)))))))

(defun stream-transform (procedure stream)
  (if (stream-endp stream)
      'empty-stream
      (stream-cons (funcall procedure (stream-first stream))
                   (stream-transform procedure
                                     (stream-rest stream)))))

(defun stream-member (object stream)
  (cond ((stream-endp stream) nil)
        ((equal object (stream-first stream)) t)
        (t (stream-member object (stream-rest stream)))))

(defmacro stream-remember (object variable)
  `(unless (stream-member ,object ,variable)
       (setf ,variable
	     (stream-append ,variable
			    (stream-cons ,object
					 'empty-stream)))
       ,object))

;;;; AUXILIARY PROCEDURES FOR LEXICAL ENCAPSULATION

(defmacro encapsulate (form)	;From a problem solution.
  `(let ((switch nil) (result nil))
       #'(lambda ()
	   (if switch
	       result
	     (setf switch t result ,form)))))

(defmacro expose (procedure)
    `(funcall ,procedure))