booleans.md

Booleans

The standard boolean objects for True and False are written as #true and #false. Alternatively, they can be written as #t #T and #f #F, respectively.

#t       ===  #true
#T       ===  #true
#f       ===  #false
#F       ===  #false

All the Ol objects are treated as true (❗including empty list, unlike some other dialects of Lisp❗), only #false counts as false in conditional expressions.

(if "hello" 2)    ==>  2
(if 123     3)    ==>  3
(if '()    17)    ==> 17
(if {}      1)    ==>  1
(if +inf.0  0)    ==>  0
(if #false 22)    ==>  #false

False is returned by default for functions that do not return a result value.

> (define (function x) (if (odd? x) "odd"))
> (function 100)
#false

Boolean constants evaluate to themselves, so they don't need to be quoted in programs.

'#true   ==>  #true
'#false  ==>  #false

TOC

not, boolean?, boolean=?

not

(not obj), procedure

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

(not #t)         ==>  #false
(not 3)          ==>  #false
(not (list 3))   ==>  #false
(not '())        ==>  #false
(not (list))     ==>  #false
(not 12345)      ==>  #false
(not not)        ==>  #false
(not 'sym)       ==>  #false
(not #f)         ==>  #true

boolean?

(boolean? obj), procedure

The boolean? predicate returns #true if obj is either #true or #false and returns #false otherwise.

(boolean? #f)    ==>  #true
(boolean? 0)     ==>  #false
(boolean? '())   ==>  #false
(boolean? #T)    ==>  #true
(boolean? #true) ==>  #true
(boolean? #false)==>  #true
(boolean? #null) ==>  #false

boolean=?

(boolean=? boolean1 boolean2 boolean3 ...), procedure

Returns #true if all the arguments are booleans and all are #true or all are #false.

(boolean=? #f #f #f #f #f 1)    ==>  #false
(boolean=? 1 #t #f #t #f #f)    ==>  #false
(boolean=? #f #t #f #t)         ==>  #false
(boolean=? #t #t #t #t)         ==>  #true
(boolean=? #f #f #f #f)         ==>  #true