available when the system starts (@pxref{Services}).
The @code{operating-system} declaration above specifies that, in
addition to the basic services, we want the @command{lshd} secure shell
-daemon listening on port 2222, and allowing remote @code{root} logins
-(@pxref{Invoking lshd,,, lsh, GNU lsh Manual}). Under the hood,
+daemon listening on port 2222 (@pxref{Networking Services,
+@code{lsh-service}}). Under the hood,
@code{lsh-service} arranges so that @code{lshd} is started with the
right command-line options, possibly with supporting configuration files
-generated as needed (@pxref{Defining Services}). @xref{operating-system
-Reference}, for details about the available @code{operating-system}
-fields.
+generated as needed (@pxref{Defining Services}).
+
+@cindex customization, of services
+@findex modify-services
+Occasionally, instead of using the base services as is, you will want to
+customize them. For instance, to change the configuration of
+@code{guix-daemon} and Mingetty (the console log-in), you may write the
+following instead of @var{%base-services}:
+
+@lisp
+(modify-services %base-services
+ (guix-service-type config =>
+ (guix-configuration
+ (inherit config)
+ (use-substitutes? #f)
+ (extra-options '("--gc-keep-outputs"))))
+ (mingetty-service-type config =>
+ (mingetty-configuration
+ (inherit config)
+ (motd (plain-file "motd" "Hi there!")))))
+@end lisp
+
+@noindent
+The effect here is to change the options passed to @command{guix-daemon}
+when it is started, as well as the ``message of the day'' that appears
+when logging in at the console. @xref{Service Reference,
+@code{modify-services}}, for more on that.
The configuration for a typical ``desktop'' usage, with the X11 display
-server, a desktop environment, network management, an SSH server, and
+server, a desktop environment, network management, power management, and
more, would look like this:
@lisp
@xref{Desktop Services}, for the exact list of services provided by
@var{%desktop-services}. @xref{X.509 Certificates}, for background
information about the @code{nss-certs} package that is used here.
+@xref{operating-system Reference}, for details about all the available
+@code{operating-system} fields.
Assuming the above snippet is stored in the @file{my-system-config.scm}
file, the @command{guix system reconfigure my-system-config.scm} command
@result{} #t
@end example
+The @code{modify-services} form provides a handy way to change the
+parameters of some of the services of a list such as
+@var{%base-services} (@pxref{Base Services, @code{%base-services}}). Of
+course, you could always use standard list combinators such as
+@code{map} and @code{fold} to do that (@pxref{SRFI-1, List Library,,
+guile, GNU Guile Reference Manual}); @code{modify-services} simply
+provides a more concise form for this common pattern.
+
+@deffn {Scheme Syntax} modify-services @var{services} @
+ (@var{type} @var{variable} => @var{body}) @dots{}
+
+Modify the services listed in @var{services} according to the given
+clauses. Each clause has the form:
+
+@example
+(@var{type} @var{variable} => @var{body})
+@end example
+
+where @var{type} is a service type, such as @var{guix-service-type}, and
+@var{variable} is an identifier that is bound within @var{body} to the
+value of the service of that @var{type}. @xref{Using the Configuration
+System}, for an example.
+
+This is a shorthand for:
+
+@example
+(map (lambda (service) @dots{}) @var{services})
+@end example
+@end deffn
+
+Next comes the programming interface for service types. This is
+something you want to know when writing new service definitions, but not
+necessarily when simply looking for ways to customize your
+@code{operating-system} declaration.
+
@deftp {Data Type} service-type
@cindex service type
This is the representation of a @dfn{service type} (@pxref{Service Types
service-kind
service-parameters
+ modify-services
service-back-edges
fold-services
(parameters service-parameters))
+(define-syntax %modify-service
+ (syntax-rules (=>)
+ ((_ service)
+ service)
+ ((_ svc (kind param => exp ...) clauses ...)
+ (if (eq? (service-kind svc) kind)
+ (let ((param (service-parameters svc)))
+ (service (service-kind svc)
+ (begin exp ...)))
+ (%modify-service svc clauses ...)))))
+
+(define-syntax modify-services
+ (syntax-rules ()
+ "Modify the services listed in SERVICES according to CLAUSES. Each clause
+must have the form:
+
+ (TYPE VARIABLE => BODY)
+
+where TYPE is a service type, such as 'guix-service-type', and VARIABLE is an
+identifier that is bound within BODY to the value of the service of that
+TYPE. Consider this example:
+
+ (modify-services %base-services
+ (guix-service-type config =>
+ (guix-configuration
+ (inherit config)
+ (use-substitutes? #f)
+ (extra-options '(\"--gc-keep-derivations\"))))
+ (mingetty-service-type config =>
+ (mingetty-configuration
+ (inherit config)
+ (motd (plain-file \"motd\" \"Hi there!\")))))
+
+It changes the configuration of the GUIX-SERVICE-TYPE instance, and that of
+all the MINGETTY-SERVICE-TYPE instances.
+
+This is a shorthand for (map (lambda (svc) ...) %base-services)."
+ ((_ services clauses ...)
+ (map (lambda (service)
+ (%modify-service service clauses ...))
+ services))))
\f
;;;
HCoop / jackhill / guix / guix.git / commitdiff