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>

1 files changed, 33 insertions, 5 deletions

diff --git a/gnu/services/configuration.scm b/gnu/services/configuration.scm
index ecc5049a79..aa5fb832d5 100644
--- a/gnu/services/configuration.scm
+++ b/gnu/services/configuration.scm
@@ -42,6 +42,7 @@
   #: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-configuration/no-serialization
             no-serialization
 
+            empty-serializer?
+            tfilter-maybe-value
+            base-transducer
+
             serialize-configuration
             define-maybe
             define-maybe/no-serialization
@@ -125,13 +130,36 @@ does not have a default value" field kind)))
   (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."