What does `when` return when the condition is false?

Question

scheme@(guile-user)> (define val (when #f 1))
scheme@(guile-user)> val
scheme@(guile-user)> (null? val)
$6 = #f
scheme@(guile-user)> (boolean? val)
$7 = #f
scheme@(guile-user)> (pair? val)
$8 = #f
scheme@(guile-user)> (when val 1)
$9 = 1

It does evaluate to #t but what is it?

Answer 1

update: the docs says: "When ... the test evaluates to #f, the value of the expression is not specified." So it might be anything, and shouldn't be relied upon for anything.

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It returns a value which is not null?, not boolean? and not pair?:

scheme@(guile-user)> (null? val)
$6 = #f
scheme@(guile-user)> (boolean? val)
$7 = #f
scheme@(guile-user)> (pair? val)
$8 = #f

It is not an #f, as evidenced by

scheme@(guile-user)> (when val 1)
$9 = 1

And it prints as nothing,

scheme@(guile-user)> val
scheme@(guile-user)> 

So what is it? A value is defined by its interactions. Its internal representation in a specific implementation is not that important.

Short answer is, it is a "not a value". (sic)

Chez Scheme prints it as #<void>:

(define val (when #f 1))
(display val)
; Output:
#<void>

And Guile 2.0.13 at ideone.com prints it as #<unspecified>.