+(define (instantiate-missing-services services)
+ "Return SERVICES, a list, augmented with any services targeted by extensions
+and missing from SERVICES. Only service types with a default value can be
+instantiated; other missing services lead to a
+'&missing-target-service-error'."
+ (define (adjust-service-list svc result instances)
+ (fold2 (lambda (extension result instances)
+ (define target-type
+ (service-extension-target extension))
+
+ (match (vhash-assq target-type instances)
+ (#f
+ (let ((default (service-type-default-value target-type)))
+ (if (eq? &no-default-value default)
+ (missing-target-error svc target-type)
+ (let ((new (service target-type)))
+ (values (cons new result)
+ (vhash-consq target-type new instances))))))
+ (_
+ (values result instances))))
+ result
+ instances
+ (service-type-extensions (service-kind svc))))
+
+ (let loop ((services services))
+ (define instances
+ (fold (lambda (service result)
+ (vhash-consq (service-kind service) service
+ result))
+ vlist-null services))
+
+ (define adjusted
+ (fold2 adjust-service-list
+ services instances
+ services))
+
+ ;; If we instantiated services, they might in turn depend on missing
+ ;; services. Loop until we've reached fixed point.
+ (if (= (length adjusted) (vlist-length instances))
+ adjusted
+ (loop adjusted))))
+