3.12 Void and Undefined

Some procedures or expression forms have no need for a result value. For example, the display procedure is called only for the side-effect of writing output. In such cases the result value is normally a special constant that prints as #<void>. When the result of an expression is simply #<void>, the REPL does not print anything.

The void procedure takes any number of arguments and returns #<void>. (That is, the identifier void is bound to a procedure that returns #<void>, instead of being bound directly to #<void>.)

The undefined constant, which prints as #<undefined>, is sometimes used as the result of a reference whose value is not yet available. In previous versions of Racket (before version 6.1), referencing a local binding too early produced #<undefined>; too-early references now raise an exception, instead.

The undefined result can still be produced in some cases by the shared form.

(define (fails)   (define x x)   x)

> (fails) x: undefined;  cannot use before initialization