services: Add 'simple-service'.

* gnu/services.scm (simple-service): New procedure.
* doc/guix.texi (Service Reference): Document it.

doc/guix.texi

@@ -11348,6 +11348,25 @@ the extension; it must return a valid value for the target service.
 Return true if @var{obj} is a service extension.
 @end deffn
 
+Occasionally, you might want to simply extend an existing service.  This
+involves creating a new service type and specifying the extension of
+interest, which can be verbose; the @code{simple-service} procedure
+provides a shorthand for this.
+
+@deffn {Scheme Procedure} simple-service @var{name} @var{target} @var{value}
+Return a service that extends @var{target} with @var{value}.  This works
+by creating a singleton service type @var{name}, of which the returned
+service is an instance.
+
+For example, this extends mcron (@pxref{Scheduled Job Execution}) with
+an additional job:
+
+@example
+(simple-service 'my-mcron-job mcron-service-type
+                #~(job '(next-hour (3)) "guix gc -F 2G"))
+@end example
+@end deffn
+
 At the core of the service abstraction lies the @code{fold-services}
 procedure, which is responsible for ``compiling'' a list of services
 down to a single directory that contains everything needed to boot and

gnu/services.scm

@@ -50,6 +50,7 @@
             service-kind
             service-parameters
 
+            simple-service
             modify-services
             service-back-edges
             fold-services
@@ -141,6 +142,13 @@
   (type       service-kind)
   (parameters service-parameters))
 
+(define (simple-service name target value)
+  "Return a service that extends TARGET with VALUE.  This works by creating a
+singleton service type NAME, of which the returned service is an instance."
+  (let* ((extension (service-extension target identity))
+         (type      (service-type (name name)
+                                  (extensions (list extension)))))
+    (service type value)))
 
 (define-syntax %modify-service
   (syntax-rules (=>)