if, cond, and, and or (original) (raw)
3.12 Conditionals: if, cond, and, and or🔗ℹ
Conditionals in The Racket Guide introduces conditionals.
(if test-expr then-expr else-expr)
Evaluates test-expr. If it produces any value other than#f, then then-expr is evaluated, and its results are the result for the if form. Otherwise, else-expr is evaluated, and its results are the result for the ifform. The then-expr and else-expr are in tail position with respect to the if form.
Examples:
(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 lastcond-clause.
If no cond-clauses are present, the result is #.
If only a [else then-body ...+] is present, then thethen-bodys are evaluated. The results from all but the lastthen-body are ignored. The results of the lastthen-body, which is in tail position with respect to thecond form, are the results for the whole condform.
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 lastthen-body are ignored. The results of the lastthen-body, which is in tail position with respect to thecond form, provides the result for the whole condform.
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.
The result of the test-expr is returned as the result of the cond form. Thetest-expr is not in tail position.
Examples:
> (cond) 5 'here '(-2 -3) '(2 3)
Recognized specially within forms like cond. Anelse form as an expression is a syntax error.
Recognized specially within forms like cond. A=> form as an expression is a syntax error.
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 theexpr.
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 andexpression with the remaining exprs in tail position with respect to the original and form.
Examples:
> (and) #t > (and 1) 1 > (and (values 1 2)) > (and #f (error "doesn't get here")) #f > (and #t 5) 5
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 theexpr.
Otherwise, the first expr is evaluated. If it produces a value other than #f, that result is the result of theor expression. Otherwise, the result is the same as anor expression with the remaining exprs in tail position with respect to the original or form.
Examples:
> (or) #f > (or 1) 1 > (or (values 1 2)) > (or 5 (error "doesn't get here")) 5 > (or #f 5) 5