Private system on ASDF

foo.asd

;; -*- mode: lisp -*-

(defpackage :foo.asdf
  (:use :cl :asdf :uiop)
  (:export #:hideable-system))
(in-package :foo.asdf)

(eval-when (:compile-toplevel :load-toplevel :execute)
  (defclass hideable-system (system)
    ((private :initform nil :initarg :private :reader private-p))
    (:documentation "If PRIVATE is true, ASDF signals an error when one tries to OPERATE this system."))

  (defmethod operate :around (operation (component hideable-system) &key &allow-other-keys)
    (if (private-p component)
        (error "Private system ~S cannot be directly operated." (component-name component))
        (call-next-method))))

(defsystem "foo"
  :version "0.0.1"
  :author "Hugo I."
  :license "public domain"
  :depends-on ("foo/private")
  :components ((:file "main")))

(defsystem "foo/private"
  :pathname "private"
  :class "FOO.ASDF:HIDEABLE-SYSTEM"
  :private t
  :components ((:file "package")))