[plt-scheme] Singleton pattern using class.ss

Thank  you!

Yes it does help.

I'm trying to work out the class.ss equivalent of java class methods and
fields like this;

public class ActivityManager extends Thread {
> [...]
>    private synchronized static ActivityManager getInstance() {
>        if ( s_instance == null || !s_instance.isAlive() ) {
>            s_instance = new ActivityManager();
>        }
>        return s_instance;
>    }
>
[code from prefuse.org; I'm porting bits to scheme]

Do you mind if I put your example in the scheme cookbook?
(in or near http://schemecookbook.org/Cookbook/IntroductionToMzlibClasses )

The 'Factory Pattern' is next for me. Any suggestions gratefully accepted.

Cheers,


Stephen





On Dec 11, 2007 2:58 AM, Matthias Felleisen <matthias at ccs.neu.edu> wrote:
> On Dec 10, 2007, at 8:48 PM, Stephen De Gabrielle wrote:
>
> > Hi, I'm trying to replicate the 'singleton' pattern using class.ss,
> > but I'm having a little trouble. Can anyone point of to an example?
>
>
> One of the things you can do is instantiate a class once and export
> only that instance:
>
> (module singleton mzscheme
>
>    (require (lib "class.ss"))
>
>    (define one%
>      (class object%
>        (super-new)
>        (define/public (hello) one))
>
>    (define one (new one%))
>
>    (provide one))
>
> Inside the class you never use new; instead you refer to this
> instance. Does this help?
>
> -- Matthias
>
>
>



-- 
Cheers,

Stephen



--
Stephen De Gabrielle
s.degabrielle at ucl.ac.uk
Telephone +44 (0)20 7679 5242 (x45242)
Mobile                  079 851 890 45
http://www.uclic.ucl.ac.uk/annb/MaSI.html
University College London Interaction Centre
Remax House - 31/32 Alfred Place
London - WC1E 7DP
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