bug#56799: [PATCH 3/5] services: configuration: Add maybe-value exported procedure.

[Attila Lendvai <attila@lendvai.name>]

* gnu/services/configuration.scm (maybe-value): New procedure.
---
 gnu/services/configuration.scm | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/gnu/services/configuration.scm b/gnu/services/configuration.scm
index a9426066b9..60965486a7 100644
--- a/gnu/services/configuration.scm
+++ b/gnu/services/configuration.scm
@@ -58,6 +58,7 @@ (define-module (gnu services configuration)
             define-maybe
             define-maybe/no-serialization
             %unset-value
+            maybe-value
             maybe-value-set?
             generate-documentation
             configuration->documentation
@@ -315,6 +316,15 @@ (define (maybe-value-set? value)
   "Predicate to check whether a 'maybe' value was explicitly provided."
   (not (eq? %unset-value value)))
 
+;; Ideally there should be a compiler macro for this predicate, that expands
+;; to a conditional that only instantiates the default value when needed.
+(define* (maybe-value value #:optional (default #f))
+  "Returns VALUE, unless it is the unset value, in which case it returns
+DEFAULT."
+  (if (maybe-value-set? value)
+      value
+      default))
+
 ;; A little helper to make it easier to document all those fields.
 (define (generate-documentation documentation documentation-name)
   (define (str x) (object->string x))
-- 
2.35.1