typesetting/quad/qtest/mds/void-and-undef.md

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>.)

Examples:

> (void)       
> (void 1 2 3) 
> (list (void))
'(#<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