;;;
;;; singleton.scm - implements singleton mixin
;;;
;;;   Copyright (c) 2000-2025  Shiro Kawai  <shiro@acm.org>
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(define-module gauche.mop.singleton
  (use gauche.threads)
  (export <singleton-meta> <singleton-mixin> instance-of))
(select-module gauche.mop.singleton)

(define-class <singleton-meta> (<class>)
  ((%the-singleton-instance :init-form (atom #f)))
  )

(define-method make ((class <singleton-meta>) . initargs)
  (atomic-update!
   (slot-ref class '%the-singleton-instance)
   (^[x] (or x (next-method)))))

(define-method instance-of ((class <singleton-meta>) . initargs)
  (apply make class initargs))

;; convenience mixin class.  you can either inherit <singleton-mixin>,
;; or specifying :metaclass <singleton-meta> to your class.
(define-class <singleton-mixin> ()
  ()
  :metaclass <singleton-meta>)
