[bug#57168] [PATCH 01/14] services: configuration: Add a 'maybe-value-set?' procedure.

[Maxim Cournoyer]

* 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.
---
 doc/guix.texi                  | 7 ++++++-
 gnu/services/configuration.scm | 5 +++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/doc/guix.texi b/doc/guix.texi
index 86cfe7d49c..039df29ebc 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -38999,7 +38999,7 @@ 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
 
@@ -39030,6 +39030,11 @@ 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
diff --git a/gnu/services/configuration.scm b/gnu/services/configuration.scm
index 3007e8de35..b41b4d2e62 100644
--- a/gnu/services/configuration.scm
+++ b/gnu/services/configuration.scm
@@ -57,6 +57,7 @@ (define-module (gnu services configuration)
             serialize-configuration
             define-maybe
             define-maybe/no-serialization
+            maybe-value-set?
             generate-documentation
             configuration->documentation
             empty-serializer
@@ -300,6 +301,10 @@ (define-configuration stem (field field-type+def
 (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))
-- 
2.36.1