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 reslt 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>.)

Examples:

> (void)

> (void 1 2 3)

> (list (void))

(#<void>)

A constant that prints as #<undefined> is used as the result of a reference to a local binding when the binding is not yet initialized. Such early references are not possible for bindings that corerspond to procedure arguments, let bindings, or let* bindings; early reference requires a recursive binding context, such as letrec or local defines in a procedure body. Also, early references to top-level and module-level bindings raise an exception, instead of producing #<undefined>. For these reasons, #<undefined> rarely appears.

(define (strange)

(define x x)

x)

> (strange)

#<undefined>