The two boolean values are #t
for true and #f
for false.
They can also be written as #true
and #false
, as per R7RS.
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 Predicates).
(<= 3 8) ⇒ #t (<= 3 -3) ⇒ #f (equal? "house" "houses") ⇒ #f (eq? #f #f) ⇒ #t
In test condition contexts like if
and cond
(see Simple Conditional Evaluation), 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).
In C, the two Scheme boolean values are available as the two constants
SCM_BOOL_T
for #t
and SCM_BOOL_F
for #f
.
Care must be taken with the false value SCM_BOOL_F
: it is not
false when used in C conditionals. In order to test for it, use
scm_is_false
or scm_is_true
.
Return #t
if obj is either #t
or #f
, else
return #f
.
SCM
SCM_BOOL_T ¶The SCM
representation of the Scheme object #t
.
SCM
SCM_BOOL_F ¶The SCM
representation of the Scheme object #f
.
int
scm_is_true (SCM obj)
¶Return 0
if obj is #f
, else return 1
.
int
scm_is_false (SCM obj)
¶Return 1
if obj is #f
, else return 0
.
int
scm_is_bool (SCM obj)
¶Return 1
if obj is either #t
or #f
, else
return 0
.
SCM
scm_from_bool (int val)
¶Return #f
if val is 0
, else return #t
.
int
scm_to_bool (SCM val)
¶Return 1
if val is SCM_BOOL_T
, return 0
when val is SCM_BOOL_F
, else signal a ‘wrong type’ error.
You should probably use scm_is_true
instead of this function
when you just want to test a SCM
value for trueness.