Void and Undefined

MzScheme returns the unique void value -- printed as #<void> -- for expressions that have undefined results in R⁵RS. The procedure void takes any number of arguments and returns void:

• (void v ) returns void.
• (void? v) returns #t if v is void, #f otherwise.

Non-global variables that are accessible but do not yet have a value are bound to the unique undefined value, printed as #<undefined>. Such variables are created by letrec-values expressions (see section 3.5), partially-initialized objects (see Chapter 6), and partially-invoked units (see Chapter 7).