prog1, prog2 (Macro)

Syntax:

Arguments and Values:

first-form—a form; evaluated as described below.

second-form—a form; evaluated as described below.

forms—an implicit progn; evaluated as described below.

result-1—the primary value resulting from the evaluation of first-form.

result-2—the primary value resulting from the evaluation of second-form.

Description:

prog1 evaluates first-form and then forms, yielding as its only value the primary value yielded by first-form.

prog2 evaluates first-form, then second-form, and then forms, yielding as its only value the primary value yielded by first-form.

Examples:

 (setq temp 1) → 1
 (prog1 temp (print temp) (incf temp) (print temp))
▷ 1
▷ 2
→ 1
 (prog1 temp (setq temp nil)) → 2
 temp → NIL
 (prog1 (values 1 2 3) 4) → 1
 (setq temp (list 'a 'b 'c))
 (prog1 (car temp) (setf (car temp) 'alpha)) → A
 temp → (ALPHA B C)
 (flet ((swap-symbol-values (x y)
          (setf (symbol-value x)
                (prog1 (symbol-value y)
                       (setf (symbol-value y) (symbol-value x))))))
   (let ((*foo* 1) (*bar* 2))
     (declare (special *foo* *bar*))
     (swap-symbol-values '*foo* '*bar*)
     (values *foo* *bar*)))
→ 2, 1
 (setq temp 1) → 1
 (prog2 (incf temp) (incf temp) (incf temp)) → 3
 temp → 4
 (prog2 1 (values 2 3 4) 5) → 2

See Also:

multiple-value-prog1, progn

Notes:

prog1 and prog2 are typically used to evaluate one or more forms with side effects and return a value that must be computed before some or all of the side effects happen.

 (prog1 {form}*) ≡ (values (multiple-value-prog1 {form}*))
 (prog2 form1 {form}*) ≡ (let () form1 (prog1 {form}*))