mv.lisp

(defconstant mv-limit 20)

(defparameter *mv-expected* 1)

(defparameter *mv* (make-array mv-limit))

(defun mv (&rest vals)
  (do ((i 0 (1+ i))
       (vs vals (cdr vs)))
      ((or (eql i *mv-expected*) (null vs))
       (setq *mv-expected* (min i mv-limit))
       (car vals))
    (setf (aref *mv* i) (car vs))))

(defmacro mv-list (form)
  (let ((val1# (gensym)))
    `(let* ((*mv-expected* mv-limit)
            (,val1# ,form))
       (if (eql *mv-expected* mv-limit)
           (list ,val1#)
           (coerce (subseq *mv* 0 *mv-expected*)
                   'list)))))

(defun make-mv-bindings (vars val1# &aux (i 0))
  (mapcar (lambda (var)
            (prog1
                (if (zerop i)
                    `(,var ,val1#)
                    `(,var (when (< ,i *mv-expected* mv-limit)
                             (aref *mv* ,i))))
              (incf i)))
          vars))

(defmacro mv-bind (vars expr &body body)
  (let ((val1# (gensym)))
    `(let* ((*mv-expected* ,(length vars))
            (,val1# ,expr))
       (let ,(make-mv-bindings vars val1#)
         ,@body))))