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prog 1 ⇒ first-form {form}*
result-1
prog 2 ⇒ first-form second-form {form}*
result-2
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.
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.
(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
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}*))
Next: progn, Previous: prog, Up: Data and Control Flow Dictionary