GNU bug report logs - #54674
[PATCH] services: configuration: Use *unspecified* instead of 'disabled.

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From: Maxime Devos <maximedevos <at> telenet.be>
To: Attila Lendvai <attila <at> lendvai.name>, 54674 <at> debbugs.gnu.org
Subject: [bug#54674] [PATCH] services: configuration: Use *unspecified* instead of 'disabled.
Date: Fri, 01 Apr 2022 21:56:58 +0200

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Attila Lendvai schreef op vr 01-04-2022 om 21:19 [+0200]:
+    		       (if (unspecified? (syntax->datum def))
     			   #`(#,name #,getter)
     			   #`(#,name #,getter (default #,def))))

I'm not sure this does what you want it to do:

(define-syntax foo
  (lambda (s)
    (syntax-case s ()
      ((_ bar)
       (pk (syntax->datum #'bar) (unspecified? (syntax->datum #'bar)))))))
(foo *unspecified*)
;;; (*unspecified* #f)

The problem is that, from the macro systems POV, *unspecified* is just
a variable reference to the global *unspecified* --- *unspecified*
isn't magic to the reader, the reader just interprets it as a symbol.

Suggestion:

(define-syntax foo2
  (lambda (s)
    (syntax-case s ()
      ((_ bar)
       (pk (syntax->datum #'bar)
           (and (identifier? #'bar)
                (free-identifier=? #'*unspecified* #'bar)))))))
(foo2 *unspecified*)
;;; (*unspecified* #t)

Greetings,
Maxime.

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