Node:Booleans, Next:Numbers, Up:Simple Data Types
The two boolean values are #t
for true and #f
for false.
Boolean values are returned by predicate procedures, such as the general
equality predicates eq?
, eqv?
and equal?
(see Equality) and numerical and string comparison operators like
string=?
(see String Comparison) and <=
(see Comparison).
(<= 3 8) => #t (<= 3 3) => #f (equal? "house" "houses") => #f (eq? #f #f) => #t
In test condition contexts like if
and cond
(see if cond case), where a group of subexpressions will be evaluated only if a
condition expression evaluates to "true", "true" means any
value at all except #f
.
(if #t "yes" "no") => "yes" (if 0 "yes" "no") => "yes" (if #f "yes" "no") => "no"
A result of this asymmetry is that typical Scheme source code more often
uses #f
explicitly than #t
: #f
is necessary to
represent an if
or cond
false value, whereas #t
is
not necessary to represent an if
or cond
true value.
It is important to note that #f
is not equivalent to any
other Scheme value. In particular, #f
is not the same as the
number 0 (like in C and C++), and not the same as the "empty list"
(like in some Lisp dialects).
The not
procedure returns the boolean inverse of its argument:
not x  Scheme Procedure 
scm_not (x)  C Function 
Return #t iff x is #f , else return #f .

The boolean?
procedure is a predicate that returns #t
if
its argument is one of the boolean values, otherwise #f
.
boolean? obj  Scheme Procedure 
scm_boolean_p (obj)  C Function 
Return #t iff obj is either #t or #f .
