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The *boolean objects* are *true* and *false*. The boolean
constant true is written as ‘`#t`’, and the boolean constant false is
written as ‘`#f`’.

The primary use for boolean objects is in the conditional expressions
`if`

, `cond`

, `and`

, and `or`

; the behavior of these
expressions is determined by whether objects are true or false. These
expressions count only `#f`

as false. They count everything else,
including `#t`

, pairs, symbols, numbers, strings, vectors, and
procedures as true (but see True and False).

Programmers accustomed to other dialects of Lisp should note that Scheme
distinguishes `#f`

and the empty list from the symbol `nil`

.
Similarly, `#t`

is distinguished from the symbol `t`

. In
fact, the boolean objects (and the empty list) are not symbols at all.

Boolean constants evaluate to themselves, so you don’t need to quote them.

#t ⇒ #t #f ⇒ #f '#f ⇒ #f t error→ Unbound variable

- variable:
**false** - variable:
**true** These variables are bound to the objects

`#f`

and`#t`

respectively. The compiler, given the`usual-integrations`

declaration, replaces references to these variables with their respective values.Note that the symbol

`true`

is not equivalent to`#t`

, and the symbol`false`

is not equivalent to`#f`

.

- standard procedure:
**boolean?***object* -
Returns

`#t`

if`object`is either`#t`

or`#f`

; otherwise returns`#f`

.(boolean? #f) ⇒ #t (boolean? 0) ⇒ #f

- standard procedure:
**not***object* - procedure:
**false?***object* -
These procedures return

`#t`

if`object`is false; otherwise they return`#f`

. In other words they*invert*boolean values. These two procedures have identical semantics; their names are different to give different connotations to the test.(not #t) ⇒ #f (not 3) ⇒ #f (not (list 3)) ⇒ #f (not #f) ⇒ #t

- extended standard procedure:
**procedure***boolean=? boolean1 boolean2 boolean3 …* -
This predicate is true iff the

`boolean`args are either all true or all false.Implementation note: The standard requires this procedure’s arguments to satisfy

`boolean?`

, but MIT/GNU Scheme allows any object to be an argument.

- procedure:
**boolean/and***object …* This procedure returns

`#t`

if none of its arguments are`#f`

. Otherwise it returns`#f`

.

- procedure:
**boolean/or***object …* This procedure returns

`#f`

if all of its arguments are`#f`

. Otherwise it returns`#t`

.

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