From: Taylan Kammer <taylan.kammer@gmail.com>
To: Maxime Devos <maximedevos@telenet.be>,
Damien Mattei <damien.mattei@gmail.com>,
Matt Wette <matt.wette@gmail.com>
Cc: guile-user <guile-user@gnu.org>
Subject: Re: Fwd: new function
Date: Wed, 22 Sep 2021 21:12:42 +0200 [thread overview]
Message-ID: <4f267793-3076-72d3-214b-077dbcf3910c@gmail.com> (raw)
In-Reply-To: <52130eefb713942a36aa5717ef4c916173567ce3.camel@telenet.be>
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
prev parent reply other threads:[~2021-09-22 19:12 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <CADEOadfF+rR=fW0fS-PkQkDN8EHFB1sJnkeePMJyc23m9shLgA@mail.gmail.com>
2021-09-19 9:18 ` Fwd: new function Damien Mattei
2021-09-19 10:38 ` Maxime Devos
2021-09-19 14:45 ` Matt Wette
2021-09-19 14:56 ` Maxime Devos
2021-09-21 13:04 ` Damien Mattei
2021-09-21 19:03 ` Maxime Devos
2021-09-22 7:52 ` Damien Mattei
2021-09-22 18:03 ` Maxime Devos
2021-09-22 19:12 ` Taylan Kammer [this message]
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