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