+ (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 (G_ "~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))
+
+
+\f
+;;;
+;;; Helpers.
+;;;
+
+(define service-parameters
+ ;; Deprecated alias.
+ service-value)