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prog Data and Control Flow Dictionary progn
Macro

prog1, prog2

prog1 first-form {form}* ⇒ result-1

prog2 first-form second-form {form}* ⇒ result-2

Arguments and Values

first-forma form; evaluated as described below.
second-forma form; evaluated as described below.
formsan implicit progn; evaluated as described below.
result-1the primary value resulting from the evaluation of first-form.
result-2the 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}*))

Backlinks

Data and Control Flow Dictionary, multiple-value-prog1, progn, Symbols in the COMMON-LISP Package