prog1, prog2 [Macro]

prog 1 => first-form {form}{*} {result-1} prog 2 => first-form second-form {form}{*} {result-2}

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::

section multiple-value-prog1 [Special Operator] , section progn [Special Operator]

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}{*}))

Go to the first, previous, next, last section, table of contents.