[bug#63985] [PATCH v3 02/11] services: configuration: Use transducers within serialize-configuration. (was : bug#63985: [PATCH RFC 0/5] Generic INI serializer & SRFI-171 for define-configuration)

[Bruno Victal <mirai@makinata.eu>]

On 2023-10-02 18:25, Maxim Cournoyer wrote:
> Bruno Victal <mirai@makinata.eu> writes:
>> diff --git a/gnu/services/telephony.scm b/gnu/services/telephony.scm
>> index 23ccb8d403..56b7772f58 100644
>> --- a/gnu/services/telephony.scm
>> +++ b/gnu/services/telephony.scm
>> @@ -204,22 +205,20 @@ (define (jami-account->alist jami-account-object)
>>        ('rendezvous-point? "Account.rendezVous")
>>        ('peer-discovery? "Account.peerDiscovery")
>>        ('bootstrap-hostnames "Account.hostname")
>> -      ('name-server-uri "RingNS.uri")
>> -      (_ #f)))
>> +      ('name-server-uri "RingNS.uri")))
>>  
>> -  (filter-map (lambda (field)
>> -                (and-let* ((name (field-name->account-detail
>> +  (define jami-account-transducer
>> +    (compose (tremove empty-serializer?)
>> +             (tfilter-maybe-value jami-account-object)
>> +             (tmap (lambda (field)
>> +                     (let* ((name (field-name->account-detail
>>                                    (configuration-field-name field)))
>> -                           (value ((configuration-field-serializer field)
>> -                                   name ((configuration-field-getter field)
>> -                                         jami-account-object)))
>> -                           ;; The define-maybe default serializer produces an
>> -                           ;; empty string for unspecified values.
>> -                           (value* (if (string-null? value)
>> -                                       #f
>> -                                       value)))
>> -                  (cons name value*)))
>> -              jami-account-fields))
>> +                            (value ((configuration-field-serializer field)
>> +                                    name ((configuration-field-getter field)
>> +                                          jami-account-object))))
>> +                       (cons name value))))))
>> +
>> +  (list-transduce jami-account-transducer rcons jami-account-fields))
> 
> Could you please state in a comment under "(define
> jami-account-transducer" why the base transducer doesn't suffice?  It
> isn't obvious to me from a casual inspection.  I guess it's because
> base-transducer is not recursive?  Should it be?

This is because you're changing the field names of the JAMI-ACCOUNT record type
through `field-name->account-detail'.
Conventional serializers are (serializer (field-name value)) procedures and this is
how `base-transducer' calls them. Here you want to do something of the sort
(serializer (translate-field-name name) value) so a custom transducer was written to
account for this detour.

I wonder if we could simply this with some functional programming as discussed in
[1] instead.

>>  (test-equal "symbol maybe value serialization, unspecified"
>>    ""
>> -  (gexp->approximate-sexp
>> +  (eval-gexp
>>     (serialize-configuration (config-with-maybe-symbol)
>>                              config-with-maybe-symbol-fields)))
> 
> That's nice, though I don't understand why gexp->approximate needs to be
> turned into eval-gexp?

Using `gexp->approximate-sexp' alone doesn't result in a evaluation of the
serialization process so eval-gexp has to be used in order to actually perform
this test.


[1]: Message-ID: <673081be-14c1-4864-9bd1-1cbc908823a6@makinata.eu>
     Link: <https://lists.gnu.org/archive/html/guix-patches/2023-10/msg00313.html>

-- 
Furthermore, I consider that nonfree software must be eradicated.

Cheers,
Bruno.