services: Add 'simple-service'.

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

1 files changed, 8 insertions, 0 deletions

diff --git a/gnu/services.scm b/gnu/services.scm
index 5479bfae19..7e322c50b0 100644
--- a/gnu/services.scm
+++ b/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 (=>)