Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!apple!bloom-beacon!ai-lab!jba
From: jba@wheaties.ai.mit.edu (Jonathan Amsterdam)
Newsgroups: alt.sources
Subject: AAL sources (3 of 8)
Message-ID: <2914@wilde.ai.mit.edu>
Date: 10 Jun 89 21:29:38 GMT
Distribution: alt
Organization: MIT AI Lab, Cambridge, MA
Lines: 84


;;; -*- Mode: LISP; Syntax: Common-lisp; Package: USER; Base: 10 -*-

;;; Streams.
;;; Copyright 1988 by Jonathan Amsterdam.  All Rights Reserved.

(provide 'streams)

(defconstant *empty-stream* nil)

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Macros.

(defmacro delay (thing)
  `#'(lambda () ,thing))


(defmacro dostream ((var stream) &body body)
  ;; Iterate through the elements of a stream.  Syntax is like dolist.
  (let ((tempvar (gensym)))
    `(do* ((,tempvar ,stream (stream-cdr ,tempvar))
	   (,var (stream-car ,tempvar) (stream-car ,tempvar)))
	  ((stream-empty? ,tempvar) *empty-stream*)
       ,@body)))

(defmacro stream-cons (thing stream)
  `(cons ,thing (delay ,stream)))

(defmacro stream-append (stream1 stream2)
  `(stream-append-func ,stream1 (delay ,stream2)))

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Functions.

(defun force (thing)
  (funcall thing))

(defun stream-append-func (stream delayed-stream)
  (if (stream-empty? stream)
      (force delayed-stream)
      (stream-cons (stream-car stream)
		   (stream-append-func (stream-cdr stream) delayed-stream))))

(defun stream-mapcar (function stream)
  (if (stream-empty? stream)
      *empty-stream*
      (stream-cons (funcall function (stream-car stream))
		   (stream-mapcar function (stream-cdr stream)))))

(defun stream-mapcan (function stream)
  (if (stream-empty? stream)
      *empty-stream*
      (stream-append (funcall function (stream-car stream))
		     (stream-mapcan function (stream-cdr stream)))))

(defun stream-empty? (stream)
  (eq stream *empty-stream*))

(defun stream-car (stream)
  (car stream))

(defun stream-cdr (stream)
  ;; This is the only function besides stream-cons and stream-append that
  ;; differs from normal list functions.
  (force (cdr stream)))

(defun stream->list (stream)
  (if (stream-empty? stream)
      nil
      (cons (stream-car stream)
	    (stream->list (stream-cdr stream)))))

(defun list->stream (list)
  (if (null list)
      *empty-stream*
      (stream-cons (car list)
		   (list->stream (cdr list)))))

(defun stream (&rest things)
  ;; This is like the list function; it returns a stream of its arguments.
  (list->stream (copy-list things)))	;Must copy in Symbolics Common Lisp,
					;because the rest arg is popped from
					;the stack after stream returns.

;;; End streams.lisp.