3.12 Conditionals: if, cond, and, and or
Conditionals in The Racket Guide introduces conditionals.
syntax
(if test-expr then-expr else-expr)
> (if (positive? -5) (error "doesn't get here") 2) 2
> (if (positive? 5) 1 (error "doesn't get here")) 1
> (if 'we-have-no-bananas "yes" "no") "yes"
syntax
(cond cond-clause ...)
cond-clause = [test-expr then-body ...+] | [else then-body ...+] | [test-expr => proc-expr] | [test-expr]
A cond-clause that starts with else must be the last cond-clause.
If no cond-clauses are present, the result is #<void>.
If only a [else then-body ...+] is present, then the then-bodys are evaluated. The results from all but the last then-body are ignored. The results of the last then-body, which is in tail position with respect to the cond form, are the results for the whole cond form.
Otherwise, the first test-expr is evaluated. If it produces #f, then the result is the same as a cond form with the remaining cond-clauses, in tail position with respect to the original cond form. Otherwise, evaluation depends on the form of the cond-clause:
[test-expr then-body ...+] The then-bodys are evaluated in order, and the results from all but the last then-body are ignored. The results of the last then-body, which is in tail position with respect to the cond form, provides the result for the whole cond form.
[test-expr => proc-expr] The proc-expr is evaluated, and it must produce a procedure that accepts one argument, otherwise the exn:fail:contract exception is raised. The procedure is applied to the result of test-expr in tail position with respect to the cond expression.
[test-expr] The result of the test-expr is returned as the result of the cond form. The test-expr is not in tail position.
> (cond)
> (cond [else 5]) 5
> (cond [(positive? -5) (error "doesn't get here")] [(zero? -5) (error "doesn't get here, either")] [(positive? 5) 'here]) 'here
> (cond [(member 2 '(1 2 3)) => (lambda (l) (map - l))]) '(-2 -3)
> (cond [(member 2 '(1 2 3))]) '(2 3)
syntax
syntax
syntax
(and expr ...)
If no exprs are provided, then result is #t.
If a single expr is provided, then it is in tail position, so the results of the and expression are the results of the expr.
Otherwise, the first expr is evaluated. If it produces #f, the result of the and expression is #f. Otherwise, the result is the same as an and expression with the remaining exprs in tail position with respect to the original and form.
syntax
(or expr ...)
If no exprs are provided, then result is #f.
If a single expr is provided, then it is in tail position, so the results of the or expression are the results of the expr.
Otherwise, the first expr is evaluated. If it produces a value other than #f, that result is the result of the or expression. Otherwise, the result is the same as an or expression with the remaining exprs in tail position with respect to the original or form.