services: configuration: Add a 'maybe-value-set?' procedure.

* gnu/services/configuration.scm (maybe-value-set?): New procedure.
* doc/guix.texi (Complex Configurations): Document it.  Remove comment showing
usage of 'maybe-string' with a default value, which doesn't make sense.

Co-authored-by: Attila Lendvai <attila@lendvai.name>

doc/guix.texi

@@ -39003,7 +39003,7 @@ to be a string, or left unspecified.
   (name
    ;; If set to a string, the `serialize-string' procedure will be used
    ;; to serialize the string.  Otherwise this field is not serialized.
-   maybe-string    ; equivalent to (maybe-string *unspecified*)
+   maybe-string
    "The name of this module."))
 @end lisp
 
@@ -39034,6 +39034,11 @@ whether its value is set or not.
 @end lisp
 @end deffn
 
+@deffn (Scheme Procedure) maybe-value-set? @var{value}
+Predicate to check whether a user explicitly specified the value of a
+maybe field.
+@end deffn
+
 @deffn {Scheme Procedure} serialize-configuration @var{configuration} @
 @var{fields}
 Return a G-expression that contains the values corresponding to the

gnu/services/configuration.scm

@@ -57,6 +57,7 @@
             serialize-configuration
             define-maybe
             define-maybe/no-serialization
+            maybe-value-set?
             generate-documentation
             configuration->documentation
             empty-serializer
@@ -142,7 +143,8 @@ does not have a default value" field kind)))
                                     (id #'stem #'serialize-maybe- #'stem))))
        #`(begin
            (define (maybe-stem? val)
-             (or (eq? val 'unset) (stem? val)))
+             (or (not (maybe-value-set? val))
+                 (stem? val)))
            #,@(if serialize?
                   (list #'(define (serialize-maybe-stem field-name val)
                             (if (stem? val)
@@ -260,11 +262,10 @@ does not have a default value" field kind)))
                       (default-value-thunk
                         (lambda ()
                           (display '#,(id #'stem #'% #'stem))
-                          (if (eq? (syntax->datum field-default)
-                                   'unset)
+                          (if (maybe-value-set? (syntax->datum field-default))
+                              field-default
                               (configuration-missing-default-value
-                               '#,(id #'stem #'% #'stem) 'field)
-                              field-default)))
+                               '#,(id #'stem #'% #'stem) 'field))))
                       (documentation doc))
                      ...))))))))
 
@@ -300,6 +301,10 @@ does not have a default value" field kind)))
 (define (empty-serializer field-name val) "")
 (define serialize-package empty-serializer)
 
+(define (maybe-value-set? value)
+  "Predicate to check whether a 'maybe' value was explicitly provided."
+  (not (eq? 'unset value)))
+
 ;; A little helper to make it easier to document all those fields.
 (define (generate-documentation documentation documentation-name)
   (define (str x) (object->string x))