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>

1 files changed, 10 insertions, 5 deletions

diff --git a/gnu/services/configuration.scm b/gnu/services/configuration.scm
index 3007e8de35..e2c4fe9998 100644
--- a/gnu/services/configuration.scm
+++ b/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))