Macro prog1, prog2

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.

7.4.0 8result-1—the primary value resulting from the evaluation of first-form.

7.4.0 11result-2—the primary value resulting from the evaluation of second-form.

Description:

7.4.0 6prog1 evaluates first-form and then forms, yielding as its only value the primary value yielded by first-form.

7.4.0 9prog2 evaluates first-form, then second-form, and then forms, yielding as its only value the primary value yielded by first-form.

I hope this is implied by the above. -kmp 22-Oct-91 The arguments are evaluated in the order in which they are given.

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

Side Effects:

None.

Affected By:

None.

Exceptional Situations:

None.

See Also:

multiple-value-prog1, progn

Notes:

7.4.0 7prog1 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.

The second example is wrong because it passes top-level-ness to form1, which is forbidden by issue MACRO-SUBFORMS-TOP-LEVEL-P. --sjl 5 Mar 92 \code (prog1 \starparam{form}) \EQ (values (multiple-value-prog1 \starparam{form})) (prog2 \param{form1} \starparam{form}) \EQ (progn \param{form1} (prog1 \starparam{form})) \endcode

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