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services: configuration: Add %unset-value exported variable.

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* gnu/services/configuration.scm (%unset-value): New variable.
(normalize-field-type+def): Use it.
(maybe-value-unset?): Use it.

Signed-off-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>
This commit is contained in:
Attila Lendvai 2022-08-24 14:40:39 +02:00 committed by Maxim Cournoyer
parent 0e11067d53
commit 147f8f19f8
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1 changed files with 13 additions and 3 deletions
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16
gnu/services/configuration.scm
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@ -57,6 +57,7 @@ (define-module (gnu services configuration)
serialize-configuration serialize-configuration
define-maybe define-maybe
define-maybe/no-serialization define-maybe/no-serialization
%unset-value
maybe-value-set? maybe-value-set?
generate-documentation generate-documentation
configuration->documentation configuration->documentation
@ -172,10 +173,10 @@ (define (normalize-field-type+def s)
(values #'(field-type def))) (values #'(field-type def)))
((field-type) ((field-type)
(identifier? #'field-type) (identifier? #'field-type)
(values #'(field-type 'unset))) (values #'(field-type %unset-value)))
(field-type (field-type
(identifier? #'field-type) (identifier? #'field-type)
(values #'(field-type 'unset))))) (values #'(field-type %unset-value)))))
(define (define-configuration-helper serialize? serializer-prefix syn) (define (define-configuration-helper serialize? serializer-prefix syn)
(syntax-case syn () (syntax-case syn ()
@ -301,9 +302,18 @@ (define-configuration stem (field field-type+def
(define (empty-serializer field-name val) "") (define (empty-serializer field-name val) "")
(define serialize-package empty-serializer) (define serialize-package empty-serializer)
;; Ideally this should be an implementation detail, but we export it
;; to provide a simpler API that enables unsetting a configuration
;; field that has a maybe type, but also a default value.
;;
;; An example use-case would be something like a network application
;; that uses a default port, but the field can explicitly be unset to
;; request a random port at startup.
(define %unset-value 'unset)
(define (maybe-value-set? value) (define (maybe-value-set? value)
"Predicate to check whether a 'maybe' value was explicitly provided." "Predicate to check whether a 'maybe' value was explicitly provided."
(not (eq? 'unset value))) (not (eq? %unset-value value)))
;; A little helper to make it easier to document all those fields. ;; A little helper to make it easier to document all those fields.
(define (generate-documentation documentation documentation-name) (define (generate-documentation documentation documentation-name)