(r5rs.info.gz) Conditional
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4.2.1 Conditionals
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-- library syntax: cond <clause1> <clause2> ...,
_Syntax:_ Each <clause> should be of the form
(<test> <expression1> ...,)
where <test> is any expression. Alternatively, a <clause> may be
of the form
(<test> => <expression>)
The last <clause> may be an "else clause," which has the form
(else <expression1> <expression2> ...,).
_Semantics:_ A `cond' expression is evaluated by evaluating the
<test> expressions of successive <clause>s in order until one of
them evaluates to a true value (see section Booleans).
When a <test> evaluates to a true value, then the remaining
<expression>s in its <clause> are evaluated in order, and the
result(s) of the last <expression> in the <clause> is(are)
returned as the result(s) of the entire `cond' expression. If the
selected <clause> contains only the <test> and no <expression>s,
then the value of the <test> is returned as the result. If the
selected <clause> uses the `=>' alternate form, then the
<expression> is evaluated. Its value must be a procedure that
accepts one argument; this procedure is then called on the value
of the <test> and the value(s) returned by this procedure is(are)
returned by the `cond' expression. If all <test>s evaluate to
false values, and there is no else clause, then the result of the
conditional expression is unspecified; if there is an else clause,
then its <expression>s are evaluated, and the value(s) of the last
one is(are) returned.
(cond ((> 3 2) 'greater)
((< 3 2) 'less)) ==> greater
(cond ((> 3 3) 'greater)
((< 3 3) 'less)
(else 'equal)) ==> equal
(cond ((assv 'b '((a 1) (b 2))) => cadr)
(else #f)) ==> 2
-- library syntax: case <key> <clause1> <clause2> ...,
_Syntax:_ <Key> may be any expression. Each <clause> should have
the form
((<datum1> ...,) <expression1> <expression2> ...,),
where each <datum> is an external representation of some object.
All the <datum>s must be distinct. The last <clause> may be an
"else clause," which has the form
(else <expression1> <expression2> ...,).
_Semantics:_ A `case' expression is evaluated as follows. <Key> is
evaluated and its result is compared against each <datum>. If the
result of evaluating <key> is equivalent (in the sense of `eqv?';
see section Equivalence predicates) to a <datum>, then the
expressions in the corresponding <clause> are evaluated from left
to right and the result(s) of the last expression in the <clause>
is(are) returned as the result(s) of the `case' expression. If
the result of evaluating <key> is different from every <datum>,
then if there is an else clause its expressions are evaluated and
the result(s) of the last is(are) the result(s) of the `case'
expression; otherwise the result of the `case' expression is
unspecified.
(case (* 2 3)
((2 3 5 7) 'prime)
((1 4 6 8 9) 'composite)) ==> composite
(case (car '(c d))
((a) 'a)
((b) 'b)) ==> _unspecified_
(case (car '(c d))
((a e i o u) 'vowel)
((w y) 'semivowel)
(else 'consonant)) ==> consonant
-- library syntax: and <test1> ...,
The <test> expressions are evaluated from left to right, and the
value of the first expression that evaluates to a false value (see
section Booleans) is returned. Any remaining expressions
are not evaluated. If all the expressions evaluate to true
values, the value of the last expression is returned. If there
are no expressions then #t is returned.
(and (= 2 2) (> 2 1)) ==> #t
(and (= 2 2) (< 2 1)) ==> #f
(and 1 2 'c '(f g)) ==> (f g)
(and) ==> #t
-- library syntax: or <test1> ...,
The <test> expressions are evaluated from left to right, and the
value of the first expression that evaluates to a true value (see
section Booleans) is returned. Any remaining expressions
are not evaluated. If all expressions evaluate to false values,
the value of the last expression is returned. If there are no
expressions then #f is returned.
(or (= 2 2) (> 2 1)) ==> #t
(or (= 2 2) (< 2 1)) ==> #t
(or #f #f #f) ==> #f
(or (memq 'b '(a b c))
(/ 3 0)) ==> (b c)
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