6.3 Booleans

The standard boolean objects for true and false are written as #t and #f. Alternatively, they can be written #true and #false, respectively. What really matters, though, are the objects that the Scheme conditional expressions (if, cond, and, or, when, unless, do) treat as true or false. The phrase “a true value” (or sometimes just “true”) means any object treated as true by the conditional expressions, and the phrase “a false value” (or “false”) means any object treated as false by the conditional expressions.

Of all the Scheme values, only #f counts as false in conditional expressions. All other Scheme values, including #t, count as true.

Note: Unlike some other dialects of Lisp, Scheme distinguishes #f and the empty list empty list from each other and from the symbol nil.

Boolean constants evaluate to themselves, so they do not need to be quoted in programs.

#t  ⇒ #t
#f  ⇒ #f
'#f ⇒ #f

procedure: not obj ¶

The not procedure returns #t if obj is false, and returns #f otherwise.

(not #t)       ⇒ #f
(not 3)        ⇒ #f
(not (list 3)) ⇒ #f
(not #f)       ⇒ #t
(not '())      ⇒ #f
(not (list))   ⇒ #f
(not 'nil)     ⇒ #f

procedure: boolean? obj ¶

The boolean? predicate returns #t if obj is either #t or #f and returns #f otherwise.

(boolean? #f)  ⇒ #t
(boolean? 0)   ⇒ #f
(boolean? '()) ⇒ #f

procedure: boolean=? boolean1 boolean2 boolean3 … ¶

Returns #t if all the arguments are #t or all are #f.