[r6rs-discuss] "Unspecified"

[Paul Schlie]

Per Bothner wrote:
> A more fundamental question is:
> (- (values 1 2)). It could be:
> (2) "spliced" yielding (- 1 2) --> -1

and (let* ([z (values 1 2)]
           [(x y) z])
          (+ x y z)) => 6

enabling multiple value use to be truly elegant and generally useful.

---

thereby for the sake of argument, all expressions denoted as arguments
are evaluated with their returned values formulating the true arguments,
where expressions returning an unspecified value are ignored, although
may be alternatively explicitly directed to return an alternate value:

(? <expr> [<opt-value>]) :: if <expr> = unspecified value
                                 returns #t or <opt-value>
                            else returns <expr> value.
(define x (if #f 0))
(define y (values 2 3))

(+ x y) :: (+ 2 3)
(+ (? x 1) y) :: (+ 1 2 3)