bug#56799: (gnu services configuration) usage of *unspecified* is problematic

[Ludovic Courtès]

Hi!

Maxim Cournoyer <maxim.cournoyer@gmail.com> skribis:

> Since commit 8cb1a49a3998c39f315a4199b7d4a121a6d66449, the
> define-configuration machinery in (gnu services configuration) uses
> *unspecified* instead of 'disabled for an unspecified field value.

As Attila wrote, the rationale as discussed in
<https://issues.guix.gnu.org/54674> was to specifically use a “special”
value without a read syntax in lieu of a symbol like 'disabled.

> While this is indeed an improvement in readability, it introduces an
> extra complication: because this new value is not self-quoting, it
> cannot be used as is in G-Exps, and values using it must be carefully
> expanded outside the gexp context, which is error prone.

Could you give a simple example of how this can happen?

In my experience, one would use ‘define-maybe’ and appropriate field
serializers such that *unspecified* never goes through.  Previously
you’d check for (eq? x 'disabled) and now you just check for
(unspecified? x).

Thanks,
Ludo’.