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authorLudovic Courtès <ludo@gnu.org>2017-04-15 23:53:23 +0200
committerLudovic Courtès <ludo@gnu.org>2017-04-16 00:48:08 +0200
commit1bb895eabf74a1e571887eb1521915e668a5c28d (patch)
tree7e8e5210a8c3f26d07ac46992f05740430436fe5 /doc
parentf816dba680124860022ba155cf5a6a337739ef11 (diff)
services: Service types can now specify a default value for instances.
* gnu/services.scm (&no-default-value): New variable. (<service-type>)[default-value]: New field. (<service>): Rename constructor from 'service' to 'make-service'. (service): New macro. (%service-with-default-value): New procedure. (&missing-value-service-error): New error condition. * tests/services.scm ("services, default value"): New test. * doc/guix.texi (Service Types and Services): Document 'default-value'. (Service Reference): Explain default values.
Diffstat (limited to 'doc')
-rw-r--r--doc/guix.texi39
1 files changed, 35 insertions, 4 deletions
diff --git a/doc/guix.texi b/doc/guix.texi
index bf46f89bf2..fdd71141f0 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -15555,11 +15555,12 @@ with a simple example, the service type for the Guix build daemon
(extensions
(list (service-extension shepherd-root-service-type guix-shepherd-service)
(service-extension account-service-type guix-accounts)
- (service-extension activation-service-type guix-activation)))))
+ (service-extension activation-service-type guix-activation)))
+ (default-value (guix-configuration))))
@end example
@noindent
-It defines two things:
+It defines three things:
@enumerate
@item
@@ -15572,6 +15573,9 @@ service, returns a list of objects to extend the service of that type.
Every service type has at least one service extension. The only
exception is the @dfn{boot service type}, which is the ultimate service.
+
+@item
+Optionally, a default value for instances of this type.
@end enumerate
In this example, @var{guix-service-type} extends three services:
@@ -15607,7 +15611,13 @@ A service of this type is instantiated like this:
The second argument to the @code{service} form is a value representing
the parameters of this specific service instance.
@xref{guix-configuration-type, @code{guix-configuration}}, for
-information about the @code{guix-configuration} data type.
+information about the @code{guix-configuration} data type. When the
+value is omitted, the default value specified by
+@code{guix-service-type} is used:
+
+@example
+(service guix-service-type)
+@end example
@var{guix-service-type} is quite simple because it extends other
services but is not extensible itself.
@@ -15670,10 +15680,31 @@ Services}). This section provides a reference on how to manipulate
services and service types. This interface is provided by the
@code{(gnu services)} module.
-@deffn {Scheme Procedure} service @var{type} @var{value}
+@deffn {Scheme Procedure} service @var{type} [@var{value}]
Return a new service of @var{type}, a @code{<service-type>} object (see
below.) @var{value} can be any object; it represents the parameters of
this particular service instance.
+
+When @var{value} is omitted, the default value specified by @var{type}
+is used; if @var{type} does not specify a default value, an error is
+raised.
+
+For instance, this:
+
+@example
+(service openssh-service-type)
+@end example
+
+@noindent
+is equivalent to this:
+
+@example
+(service openssh-service-type
+ (openssh-configuration))
+@end example
+
+In both cases the result is an instance of @code{openssh-service-type}
+with the default configuration.
@end deffn
@deffn {Scheme Procedure} service? @var{obj}