Special Operator if

Syntax:

Arguments and Values:

Test-form—a form.

Then-form—a form.

7.6.0 4Else-form—a form. The default is nil.

7.9.2 7 7.9.2 8results—if the test-form yielded true, the values returned by the then-form; otherwise, the values returned by the else-form.

Description:

if allows the execution of a form to be dependent on a single test-form.

7.6.0 3First test-form is evaluated. If the result is true, then then-form is selected; otherwise else-form is selected. Whichever form is selected is then evaluated.

Examples:

 (if t 1) → 1
 (if nil 1 2) → 2 
 (defun test ()
   (dolist (truth-value '(t nil 1 (a b c)))
     (if truth-value (print 'true) (print 'false))
     (prin1 truth-value))) → TEST
 (test)
⊳ TRUE T
⊳ FALSE NIL
⊳ TRUE 1
⊳ TRUE (A B C)
→ NIL

Affected By:

None.

Exceptional Situations:

None.

See Also:

cond, unless, when

Notes:

!!! Barmar thinks we should encourage implementations to signal an error or issue a warning if additional subforms are detected.

 (if test-form then-form else-form)
 ≡ (cond (test-form then-form) (t else-form))