[bug#36875] [PATCH] doc: Document the use of `program-file' for mcron jobs.

[Ludovic Courtès]

Hi Maxim,

Maxim Cournoyer <address@hidden> skribis:

> Ludovic Courtès <address@hidden> writes:
>
>> Hello!
>>
>> Maxim Cournoyer <address@hidden> skribis:
>>
>>> From 0fffed46b4899bf0485926399d3971a4b5e94408 Mon Sep 17 00:00:00 2001
>>> From: Maxim Cournoyer <address@hidden>
>>> Date: Thu, 1 Aug 2019 07:34:17 +0900
>>> Subject: [PATCH] doc: Document the use of `program-file' for mcron jobs.
>>>
>>> * doc/guix.texi (Scheduled Job Execution): Explain why using `program-file'
>>> for an mcron job can be necessary.  Add an example.
>>
>> [...]
>>
>>> +For more complex jobs defined in Scheme, it is safer to pass the job as a
>>> +script to mcron; otherwise, macros defined or imported with 
>>> @code{use-modules}
>>> +wouldn't expand correctly, as Guile requires macros to be strictly defined 
>>> or
>>> +imported at the top level of a Guile module.  This can be achieved using 
>>> the
>>> +@code{program-file} procedure from the @code{(guix gexp)} module, as shown 
>>> in
>>> +the example below.
>>
>> Macros are a very good example of the problem, but I wonder if it would
>> be clearer to simply write something like:
>>
>>   For more complex jobs defined in Scheme where you need control over
>>   the top level, for instance to introduce a @code{use-modules} form, you
>>   can move your code to a separate program using the @code{program-file}
>>   procedure of the @code{(guix gexp)} module (@pxref{G-Expressions}).
>>   The example below illustrates that.
>
> I like your version, which feels to me more elegant. But, from my
> experimentation, using (use-modules) in a nested form is fine for
> anything else than syntax (macros).

That’s right, but I strongly recommend not relying on non-toplevel
‘use-modules’ because (1) it’s “ugly” because it introduces new bindings
at run time, and (2) it’s not guaranteed to work in the future—in fact,
the just-released Guile 2.9.4 introduces “declarative modules”, which is
probably a first step in the direction of less run-time trickery with
modules.

> This is now live as commit 1407ebeaa1.  Thanks for feedback/review! :-)

Great, thank you!

Ludo’.