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bug#56799: [PATCH 3/5] services: configuration: Add maybe-value exported
From: |
Attila Lendvai |
Subject: |
bug#56799: [PATCH 3/5] services: configuration: Add maybe-value exported procedure. |
Date: |
Wed, 24 Aug 2022 14:40:40 +0200 |
* 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