services: configuration: Use transducers within serialize-configuration.

Introduces 'base-transducer', a SRFI-171 based transducer that can be used as a
starting point for writing custom configuration record serializing procedures.

This also fixes the symbol maybe-value serialization test case.

* gnu/services/configuration.scm (empty-serializer?): New predicate.
(base-transducer, tfilter-maybe-value): New procedure.
(serialize-configuration): Adapt to use base-transducer.

* gnu/services/telephony.scm (jami-account->alist): Use transducers to skip
fields that are unserializable or whose field maybe-value is unset.

* tests/services/configuration.scm: Remove test-expect-fail.

Signed-off-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>

gnu/services/configuration.scm

@@ -42,6 +42,7 @@ (define-module (gnu services configuration)
   #:use-module (srfi srfi-26)
   #:use-module (srfi srfi-34)
   #:use-module (srfi srfi-35)
+  #:use-module (srfi srfi-171)
   #:export (configuration-field
             configuration-field-name
             configuration-field-type
@@ -59,6 +60,10 @@ (define-module (gnu services configuration)
             define-configuration/no-serialization
             no-serialization
 
+            empty-serializer?
+            tfilter-maybe-value
+            base-transducer
+
             serialize-configuration
             define-maybe
             define-maybe/no-serialization
@@ -125,13 +130,36 @@ (define-record-type* <configuration-field>
   (default-value-thunk configuration-field-default-value-thunk)
   (documentation configuration-field-documentation))
 
+(define (empty-serializer? field)
+  "Predicate that checks whether FIELD is exempt from serialization."
+  (eq? empty-serializer
+       (configuration-field-serializer field)))
+
+(define (tfilter-maybe-value config)
+  "Return a transducer for CONFIG that removes all maybe-type fields whose
+value is '%unset-marker."
+  (tfilter (lambda (field)
+             (let ((field-value ((configuration-field-getter field) config)))
+               (maybe-value-set? field-value)))))
+
+(define (base-transducer config)
+  "Return a transducer for CONFIG that calls the serializing procedures only
+for fields marked for serialization and whose values are not '%unset-marker."
+  (compose (tremove empty-serializer?)
+           ;; Only serialize fields whose value isn't '%unset-marker%.
+           (tfilter-maybe-value config)
+           (tmap (lambda (field)
+                   ((configuration-field-serializer field)
+                    (configuration-field-name field)
+                    ((configuration-field-getter field) config))))))
+
 (define (serialize-configuration config fields)
+  "Return a G-expression that contains the values corresponding to the
+FIELDS of CONFIG, a record that has been generated by `define-configuration'.
+The G-expression can then be serialized to disk by using something like
+`mixed-text-file'."
   #~(string-append
-     #$@(map (lambda (field)
-               ((configuration-field-serializer field)
-                (configuration-field-name field)
-                ((configuration-field-getter field) config)))
-             fields)))
+     #$@(list-transduce (base-transducer config) rcons fields)))
 
 (define-syntax-rule (id ctx parts ...)
   "Assemble PARTS into a raw (unhygienic) identifier."

gnu/services/telephony.scm

@@ -37,6 +37,7 @@ (define-module (gnu services telephony)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-2)
   #:use-module (srfi srfi-26)
+  #:use-module (srfi srfi-171)
   #:use-module (ice-9 format)
   #:use-module (ice-9 match)
   #:export (jami-account
@@ -204,22 +205,20 @@ (define (field-name->account-detail name)
       ('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))
 
 (define (jami-account-list? val)
   (and (list? val)

tests/services/configuration.scm

@@ -337,13 +337,9 @@ (define-maybe symbol)
 (define-configuration config-with-maybe-symbol
   (protocol maybe-symbol ""))
 
-;;; Maybe symbol values are currently seen as serializable, because the
-;;; unspecified value is '%unset-marker%, which is a symbol itself.
-;;; TODO: Remove expected fail marker after resolution.
-(test-expect-fail 1)
 (test-equal "symbol maybe value serialization, unspecified"
   ""
-  (gexp->approximate-sexp
+  (eval-gexp
    (serialize-configuration (config-with-maybe-symbol)
                             config-with-maybe-symbol-fields)))