Before this change, using define-maybe along define-configuration with the
no-serialization syntactic keyword would result in the following warning:
warning: possibly unbound variable `VARIABLE-NAME'
This change introduces the define-maybe/no-serialization variant that does
away with defining a serialization helper procedure, which makes it possible
to avoid the above warning.
* gnu/services/configuration.scm (define-maybe/no-serialization): New syntax.
(define-maybe-helper): New procedure.
(define-maybe): Define syntax using the above procedure.
* tests/services/configuration.scm (tests): Fix module name.
(custom-number-serializer): Do not print to standard output.
(maybe-number?, serialize-maybe-number): New procedures defined via the
define-maybe macro.
(config-with-maybe-number): New configuration.
(serialize-number): New procedure.
("maybe value serialization"): New test.
(maybe-string?): New procedure defined via the define-maybe/no-serialization
macro.
(config-with-maybe-string/no-serialization): New configuration.
("maybe value without serialization no procedure bound"): New test.
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