services: Add 'simple-service'.
* gnu/services.scm (simple-service): New procedure. * doc/guix.texi (Service Reference): Document it.
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@ -11348,6 +11348,25 @@ the extension; it must return a valid value for the target service.
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Return true if @var{obj} is a service extension.
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@end deffn
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Occasionally, you might want to simply extend an existing service. This
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involves creating a new service type and specifying the extension of
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interest, which can be verbose; the @code{simple-service} procedure
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provides a shorthand for this.
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@deffn {Scheme Procedure} simple-service @var{name} @var{target} @var{value}
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Return a service that extends @var{target} with @var{value}. This works
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by creating a singleton service type @var{name}, of which the returned
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service is an instance.
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For example, this extends mcron (@pxref{Scheduled Job Execution}) with
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an additional job:
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@example
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(simple-service 'my-mcron-job mcron-service-type
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#~(job '(next-hour (3)) "guix gc -F 2G"))
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@end example
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@end deffn
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At the core of the service abstraction lies the @code{fold-services}
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procedure, which is responsible for ``compiling'' a list of services
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down to a single directory that contains everything needed to boot and
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@ -50,6 +50,7 @@
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service-kind
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service-parameters
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simple-service
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modify-services
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service-back-edges
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fold-services
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@ -141,6 +142,13 @@
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(type service-kind)
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(parameters service-parameters))
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(define (simple-service name target value)
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"Return a service that extends TARGET with VALUE. This works by creating a
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singleton service type NAME, of which the returned service is an instance."
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(let* ((extension (service-extension target identity))
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(type (service-type (name name)
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(extensions (list extension)))))
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(service type value)))
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(define-syntax %modify-service
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(syntax-rules (=>)
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