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test-expression ::= expression consequent ::= expression alternate ::= expression
An if
expression is evaluated as follows:
first, test-expression is
evaluated. If it yields a true value, then consequent is
evaluated and its values are returned. Otherwise alternate is
evaluated and its values are returned. If test yields #f
and no alternate is specified, then the result of the expression
is unspecified.
(if (> 3 2) 'yes 'no) ⇒ yes (if (> 2 3) 'yes 'no) ⇒ no (if (> 3 2) (- 3 2) (+ 3 2)) ⇒ 1 (if #f #f) ⇒ unspecified
The consequent and alternate expressions are in tail
context if the if
expression itself is.
(else
expression…)
cond-clause ::=(
test-expression body)
|(
test=>
expression)
A cond
expression is evaluated by evaluating the test-expressions
of successive cond-clauses in order until one of them
evaluates to a true value. When a test-expression evaluates to a true
value, then the remaining expressions in its cond-clause
are evaluated in order, and the results of the last expression in
the cond-clause are returned as the results of the entire
cond
expression.
If the selected cond-clause contains only the test-expression and no
expressions, then the value of the test-expression is returned as the
result. If the selected cond-clause uses the =>
alternate
form, then the expression is evaluated. Its value must be a
procedure. This procedure should accept one argument; it is called on
the value of the test-expression and the values returned by this procedure
are returned by the cond
expression.
If all test-expressions evaluate to #f
, and there is no else
clause, then the conditional expression returns unspecified values; if
there is an else
clause, then its expressions are
evaluated, and the values of the last one are returned.
(cond ((> 3 2) 'greater) ((< 3 2) 'less)) ⇒ greater (cond ((> 3 3) 'greater) ((< 3 3) 'less) (else 'equal)) ⇒ equal (cond ('(1 2 3) => cadr) (else #f)) ⇒ 2
For a cond-clause of one of the following forms:
(test expression*) (else expression expression*)
the last expression is in tail context if the cond
form
itself is. For a cond clause of the form:
(test => expression)
the (implied) call to the procedure that results from the evaluation of
expression is in tail context if the cond
form itself
is.
case-key ::= expression case-clause ::=((
datum*)
expression+)
|((
datum*)
=>
expression)
case-else-clause ::=(else
expression+)
|(else =>
expression)
Each datum is an external representation of some object.
Each datum in the entire case
expression should be distinct.
A case
expression is evaluated as follows.
eqv?
against the data represented by the datums of
each case-clause in turn, proceeding in order from left to
right through the set of clauses.
case
expression. Otherwise, the comparison process continues.
case
expression; otherwise the result of case
expression is unspecified.
If the selected case-clause or case-else-clause
uses the =>
alternate
form, then the expression is evaluated. It is an error if
its value is not a procedure accepting one argument. This
procedure is then called on the value of the key and the
values returned by this procedure are returned by the case
expression.
(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 => (lambda (x) x))) ⇒ c
The last expression of a case clause is in tail context if
the case
expression itself is.
The match
form is a generalization of case
using patterns,
match-key ::= expression match-clause ::=(
pattern [guard] body)
The match-key is evaluated, Then the match-clauses are tried in order. The first match-clause whose pattern matches (and the guard, if any, is true), is selected, and the corresponding body evaluated. It is an error if no match-clause matches.
(match value (0 (found-zero)) (x #!if (> x 0) (found-positive x)) (x #!if (< x 0) (found-negative x)) (x::symbol (found-symbol x)) (_ (found-other)))
One case
feature is not (yet) directly supported by match
:
Matching against a list of values.
However, this is easy to simulate using a guard using memq
,
memv
, or member
:
;; compare similar example under case (match (car '(c d)) (x #!if (memv x '(a e i o u)) ’vowel) (x #!if (memv x '(w y)) ’semivowel) (x x))
If there are no test-expressions, #t
is returned. Otherwise, the
test-expression are evaluated from left to right until a
test-expression returns #f
or the last test-expression is reached. In the
former case, the and
expression returns #f
without
evaluating the remaining expressions. In the latter case, the last
expression is evaluated and its values are returned.
(and (= 2 2) (> 2 1)) ⇒ #t (and (= 2 2) (< 2 1)) ⇒ #f (and 1 2 'c '(f g)) ⇒ (f g) (and) ⇒ #t
The and
keyword could be defined in terms of if
using
syntax-rules
as follows:
(define-syntax and (syntax-rules () ((and) #t) ((and test) test) ((and test1 test2 ...) (if test1 (and test2 ...) #t))))
The last test-expression is in tail context if the and
expression itself is.
If there are no test-expressions, #f
is returned. Otherwise, the
test-expressions are evaluated from left to right until a
test-expression returns a true value val or the last
test-expression is
reached. In the former case, the or
expression returns val
without evaluating the remaining expressions. In the latter case, the
last expression is evaluated and its values are returned.
(or (= 2 2) (> 2 1)) ⇒ #t (or (= 2 2) (< 2 1)) ⇒ #t (or #f #f #f) ⇒ #f (or '(b c) (/ 3 0)) ⇒ (b c)
The or
keyword could be defined in terms of if
using
syntax-rules
as follows:
(define-syntax or (syntax-rules () ((or) #f) ((or test) test) ((or test1 test2 ...) (let ((x test1)) (if x x (or test2 ...))))))
The last test-expression is in tail context if the or
expression itself is.
If test-expression is true, evaluate each form in order, returning the value of the last one.
If test-expression is false, evaluate each form in order, returning the value of the last one.
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