The
(defmacro cl:multiple-value-setq (symbols expr)
(and (or (not (consp symbols))
(eq 'quote (first symbols))
(dolist (symbol symbols)
(or (symbolp symbol) (return t))))
(error "not a list of symbols" symbols))
(setq cl:*multiple-values* nil)
(let* ((result (eval expr))
(values (if cl:*multiple-values*
*rslt*
(list result)))
(number-of-symbols (length symbols))
(number-of-values (length values))
(assignments (list 'setq)))
(dotimes (index number-of-symbols)
(push (nth index symbols) assignments)
(push (when (< index number-of-values)
(list 'quote (nth index values)))
assignments))
(setq assignments (reverse assignments))
`(progn ,assignments ',result)))
The expression is evaluated, and each symbol is assigned to the
corresponding value returned by the evaluation.
The
> (macroexpand-1 '(cl:multiple-value-setq (a b c)
(cl:values 1 2 3)))
(PROGN (SETQ A (QUOTE 1)
B (QUOTE 2)
C (QUOTE 3))
(QUOTE 1))
The quotes are necessary to prevent 'unbound variable' and 'unbound function' errors with symbols and lists.
Examples:
1. The
(defun multiple-value-function ()
(cl:values 1 2 3)) ; multiple return values
> (let ((a 'A) (b 'B) (c 'C))
(cl:multiple-value-setq (a b c)
(multiple-value-function))
(list a b c))
(1 2 3)
2. If there are no values returned in the Nyquist
*rslt* variable, the normal
function return value is assigned to the first symbol and all remaining
symbols are assigned
(defun normal-lisp-function ()
(+ 1 1)) ; normal return value
> (let ((a 'A) (b 'B) (c 'C))
(cl:multiple-value-setq (a b c)
(normal-lisp-function))
(list a b c))
(2 NIL NIL)
3. If there are less values than symbols, the extra symbols are
assigned
(defun not-enough-values-function ()
(cl:values 1 2)) ; multiple return values
> (let ((a 'A) (b 'B) (c 'C))
(cl:multiple-value-setq (a b c)
(not-enough-values-function))
(list a b c))
(1 2 NIL)
4. If there are more values than symbols, the extra values are ignored:
(defun too-many-values-function ()
(cl:values 1 2 3 4 5)) ; multiple return values
> (let ((a 'A) (b 'B) (c 'C))
(cl:multiple-value-setq (a b c)
(too-many-values-function))
(list a b c))
(1 2 3)
5. Symbols not contained in the
(defun multiple-value-function ()
(cl:values 1 2 3)) ; multiple return values
> (let ((a 'A) (b 'B) (c 'C) (d 'D) (e 'E))
(cl:multiple-value-setq (a b c)
(multiple-value-function))
(list a b c d e))
(1 2 3 D E)
5. If no bindings exist, new variables will be created:
(defun multiple-value-function ()
(cl:values 1 2 3)) ; multiple return values
> (let ((c 'C) (d 'D) (e 'E))
(cl:multiple-value-setq (a b c)
(multiple-value-function))
(list a b c d e))
(1 2 3 D E)
Caution: In the last example, two global variables 'a' and 'b'
were created, while the lexical let
variable 'c' was assigned to the
> (list a b) (1 2) > (list a b c) error: unbound variable - C
The lexical let binding of 'c' does
not exist in the global