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9.16 gauche.mop.singleton - Singleton

Module: gauche.mop.singleton

Provides a metaclass to define a singleton class.

Class: <singleton-meta>

Creates a singleton class. A singleton class is a class that is guaranteed to create only one instance. The first invocation of make creates the single instance, and further attempt of creation returns the same instance.

(define-class single () () :metaclass <singleton-meta>)

(define a (make single))
(define b (make single))

(eq? a b) ⇒ #t

The slots of the instance are initialized at the first invocation of make. Initargs of make are effective only in the fist invocation, and ignored in the subsequent invocation.

Method: instance-of (class <singleton-meta>) :rest initargs

This method just calls make with the passed arguments. It is more obvious in the program that you’re dealing with singleton.

Class: <singleton-mixin>

An instance of <singleton-meta>. Instead of specifying <singleton-meta> as the :metaclass argument of define-class, you can inherit this class to give your class the property of singleton.

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