From: Florian Gross <flgr@...>
Date: 2004-11-16T23:18:18+09:00
Subject: Re: Kernel#singleton_class

trans. (T. Onoma) wrote:

> | > | Objection to the objection: The singleton pattern is not widely used in
> | > | Ruby. Couldn't we rename the implementation of it? Maybe something like
> | > | "SingleInstanceClass"?
> | >
> | > Well, I just used it. And the rest of the OOP world knows Singleton, yes?
> |
> | What did you use it for and why would another name be a problem in your
> | case?
> 
> In current project I use it for markup BaseAdapter Subclasses. In past I've 
> used it for database pool, object pool, and CGI Singleton. There are others 
> too that I can't recall off the top of my head.

Why don't use globals (or a constant) or a Module with module functions 
instead? I don't understand how singletons are useful for pools however. 
Could you explain?

> I actually started a thread on this matter a few weeks ago, David brought up 
> some good points. The nice thing about a singleton is that it is easily 
> initialized. Pretty sure the implementation is also thread safe. Two big 
> pluses in my book. Also, from a different vantage, singletons are nothing 
> more than fully unique multitons. So are multitons also outmoded? And, if 
> what you say is so, then what do other languages, lacking Ruby's proxy class, 
> use in their stay? Should't such a "pattern" be common ground across OOPLs?

Multitons are however often immutable value objects meaning that they 
can not be used as hidden global state.

> The problem I have is very simple: constant reoccuring confusion in 
> discussions. Look back just a few post --it's even happened in this thread! 
> And it was you who once again had to explain:
> 
> <quote>
> Sorry, I was talking about the pattern that lets you only create a 
> single instance of a class. See singleton.rb in Ruby's standard library.
> </quote>
> 
> Don't you get tired of doing this? Honestly, I see this same kind of thing 
> every time singleton comes up. There was a mention of singleton in the 
> current DI thread, and guess what? I had no idea which one was meant. And 
> changing the name of the module for Singleton to SingletonPattern or 
> Pattern::Singleton is not going to help this problem at all.

Of course I'm tired of this. That's why I wanted to rename singleton.rb 
into SingleInstanceClass. (Which explains what it does in simpler terms 
than "Singleton" anyway.) It's just that the more common thing always 
deserves the simpler and shorter name. Classes that can only be 
instantiated once seem not to be useful enough (except as a way of using 
globals without using globals in which case they are an anti-pattern) to 
deserve this name. Of course this would be no problem if matz had 
decided for one of the other alternatives -- but isn't "singleton class" 
the term that is most commonly used already?