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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.
gn-latest-20200428
Ludovic Courtès 5 years ago
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commit
1bb895eabf
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  1. 39
      doc/guix.texi
  2. 62
      gnu/services.scm
  3. 11
      tests/services.scm

39
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}

62
gnu/services.scm

@ -25,6 +25,7 @@
#:use-module (guix profiles)
#:use-module (guix sets)
#:use-module (guix ui)
#:use-module ((guix utils) #:select (source-properties->location))
#:use-module (guix modules)
#:use-module (gnu packages base)
#:use-module (gnu packages bash)
@ -47,6 +48,7 @@
service-type-extensions
service-type-compose
service-type-extend
service-type-default-value
service
service?
@ -60,6 +62,9 @@
fold-services
service-error?
missing-value-service-error?
missing-value-service-error-type
missing-value-service-error-location
missing-target-service-error?
missing-target-service-error-service
missing-target-service-error-target-type
@ -119,6 +124,10 @@
(target service-extension-target) ;<service-type>
(compute service-extension-compute)) ;params -> params
(define &no-default-value
;; Value used to denote service types that have no associated default value.
'(no default value))
(define-record-type* <service-type> service-type make-service-type
service-type?
(name service-type-name) ;symbol (for debugging)
@ -132,7 +141,11 @@
;; Extend the services' own parameters with the extension composition.
(extend service-type-extend ;list of Any -> parameters
(default #f)))
(default #f))
;; Optional default value for instances of this type.
(default-value service-type-default-value ;Any
(default &no-default-value)))
(define (write-service-type type port)
(format port "#<service-type ~a ~a>"
@ -143,11 +156,53 @@
;; Services of a given type.
(define-record-type <service>
(service type value)
(make-service type value)
service?
(type service-kind)
(value service-value))
(define-syntax service
(syntax-rules ()
"Return a service instance of TYPE. The service value is VALUE or, if
omitted, TYPE's default value."
((_ type value)
(make-service type value))
((_ type)
(%service-with-default-value (current-source-location)
type))))
(define (%service-with-default-value location type)
"Return a instance of service type TYPE with its default value, if any. If
TYPE does not have a default value, an error is raised."
;; TODO: Currently this is a run-time error but with a little bit macrology
;; we could turn it into an expansion-time error.
(let ((default (service-type-default-value type)))
(if (eq? default &no-default-value)
(let ((location (source-properties->location location)))
(raise
(condition
(&missing-value-service-error (type type) (location location))
(&message
(message (format #f (_ "~a: no value specified \
for service of type '~a'")
(location->string location)
(service-type-name type)))))))
(service type default))))
(define-condition-type &service-error &error
service-error?)
(define-condition-type &missing-value-service-error &service-error
missing-value-service-error?
(type missing-value-service-error-type)
(location missing-value-service-error-location))
;;;
;;; Helpers.
;;;
(define service-parameters
;; Deprecated alias.
service-value)
@ -541,9 +596,6 @@ kernel."
;;; Service folding.
;;;
(define-condition-type &service-error &error
service-error?)
(define-condition-type &missing-target-service-error &service-error
missing-target-service-error?
(service missing-target-service-error-service)

11
tests/services.scm

@ -31,6 +31,17 @@
(test-begin "services")
(test-equal "services, default value"
'(42 123 234 error)
(let* ((t1 (service-type (name 't1) (extensions '())))
(t2 (service-type (name 't2) (extensions '())
(default-value 42))))
(list (service-value (service t2))
(service-value (service t2 123))
(service-value (service t1 234))
(guard (c ((missing-value-service-error? c) 'error))
(service t1)))))
(test-assert "service-back-edges"
(let* ((t1 (service-type (name 't1) (extensions '())
(compose +) (extend *)))

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