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

[Maxim Cournoyer]

Hi Ludo,

Ludovic Courtès <ludo@gnu.org> writes:

> 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).

Yes, I understand that.  What changed is that previously you could have
the configuration serialized and used on the service side, which is what
using *unspecified* made impossible.

Granted, few services outside of Jami probably made use of this, but it
was nevertheless a useful property.

Thanks,

Maxim