On 21.09.2021 21:03, Maxime Devos wrote:
>
> (define (foo2 set?)
> (define x) ; define an (undefined or unbound, not sure about terminology)
> variable
> (if set?
> (let ()
> (set! x 2) ; change the value of x
> (display "x=")
> (display x)
> (newline))
> 'never)
> (display x)
> (newline))
>
I didn't know (define x) without a value was possible in Guile. I guess
it's just a shorthand for (define x *unspecified*), judging by your result.
If I'm not mistaken, the only way in Scheme to get a "defined but not yet
bound" kind of situation is to use 'letrec'. If you use the 'letrec*'
variant, it guarantees a straight order of evaluation, so the following is
supposed to definitely NOT work, due to y and z being defined-not-bound:
(letrec* ((x (+ y z))
(y (random 10))
(z (random 10)))
(display x)
(newline))
However, in Guile, it seems to bind all the variables to #<unspecified>
anyway, resulting in a "wrong type argument" error (since we end up
passing #<unspecified> to '+') instead of saying that y and z are not
yet bound.
Long story short, there doesn't seem to be *any* way in Guile to have a
lexical variable that's defined but not bound.
--
Taylan
