The Traits would be cool to standardise the behaviour and to mark it as such (implicit documentation), however, Traits cannot add instance/class variables IIUC, that limits what they can do (and would make it more than one step).

On 19 Jan 2017, at 11:33, Cyril Ferlicot D. <cyril.ferlicot@gmail.com> wrote:

On 19/01/2017 11:23, Siemen Baader wrote:

...

Is there a trait to quickly create singletons in Pharo, along the way of the Ruby Singleton mixin?

https://ruby-doc.org/stdlib-1.9.3/libdoc/singleton/rdoc/Singleton.html

(and ideally also observer/observable, which I am going to need next..)

-- Siemen

Hi!

Singleton in Smalltalk are really easy so there is no Trait.

You just need a class variable #UniqueInstance and those methods:

current "Can also be named #default or #instance" ^ UniqueInstance ifNil: [ UniqueInstance := self basicNew; initialize; yourself ]

new ^ self error: 'I am a Singleton. Use current to get my instance or reset my instance with a new project. Maybe explain why there is a Singleton because we should avoid them if possible.'

Now for the observer pattern you have the Announcement framework in Pharo.

I don't really have the time to explain but I think there is some doc on internet and on the help browser of Pharo. I found this: http://pharo.gemtalksystems.com/book/LanguageAndLibraries/announcements/ (I did not read it so I don't know what it's worth)

-- Cyril Ferlicot

http://www.synectique.eu

2 rue Jacques Prévert 01, 59650 Villeneuve d'ascq France

On Thu, Jan 19, 2017 at 11:37 AM, Sven Van Caekenberghe <sven@stfx.eu> wrote:

The Traits would be cool to standardise the behaviour and to mark it as such (implicit documentation)

Exactly.

, however, Traits cannot add instance/class variables IIUC, that limits what they can do (and would make it more than one step).

Ok. So that's the difference between a Ruby mixin and a trait. That's a shame, I think what Ruby does with mixins is really nice because it a) results in less code to test, and b) makes hineritance chains more semantic - you only inherit to model spcialization, not to reuse features. Cyril, many thanks! I know how to implement it and it is true that it is not much, but I had hoped to eliminate the code (and tests!) and use a standard feature if possible ;) -- Siemen

...

On 19 Jan 2017, at 11:33, Cyril Ferlicot D. <cyril.ferlicot@gmail.com> wrote:

On 19/01/2017 11:23, Siemen Baader wrote:

...

Is there a trait to quickly create singletons in Pharo, along the way of the Ruby Singleton mixin?

https://ruby-doc.org/stdlib-1.9.3/libdoc/singleton/rdoc/Singleton.html

(and ideally also observer/observable, which I am going to need next..)

-- Siemen

Hi!

Singleton in Smalltalk are really easy so there is no Trait.

You just need a class variable #UniqueInstance and those methods:

current "Can also be named #default or #instance" ^ UniqueInstance ifNil: [ UniqueInstance := self basicNew; initialize; yourself ]

new ^ self error: 'I am a Singleton. Use current to get my instance or reset my instance with a new project. Maybe explain why there is a Singleton because we should avoid them if possible.'

Now for the observer pattern you have the Announcement framework in Pharo.

I don't really have the time to explain but I think there is some doc on internet and on the help browser of Pharo. I found this: http://pharo.gemtalksystems.com/book/LanguageAndLibraries/announcements/ (I did not read it so I don't know what it's worth)

-- Cyril Ferlicot

http://www.synectique.eu

2 rue Jacques Prévert 01, 59650 Villeneuve d'ascq France

Am 19.01.17 um 15:51 schrieb Siemen Baader:

Wouldn't a class instance variable be better? I want new singletons for every subclass. http://rmod-pharo-mooc.lille.inria.fr/MOOC/Slides/Week3/C019-W3S03-Basic-Var...

I would also have used super new, why BasicNew? But my wish to create subclasses seems to answer that already.

super new might cause problems if you also want singletons for all subclasses and at least some of their subclasses and subclasses (1st generation) throw an exception in their #new.

On Thu, Jan 19, 2017 at 3:51 PM, Siemen Baader <siemenbaader@gmail.com> wrote:

On Thu, Jan 19, 2017 at 11:33 AM, Cyril Ferlicot D. < cyril.ferlicot@gmail.com> wrote:

...

You just need a class variable #UniqueInstance and those methods:

current "Can also be named #default or #instance" ^ UniqueInstance ifNil: [ UniqueInstance := self basicNew; initialize; yourself ]

Actually this should have been: (no `;` between basicNew and initialize). ^ currentInstance ifNil: [ currentInstance := self basicNew initialize; yourself ] This is not a problem. But it illustrates that this would be nice to have it unit tested separately and mixed in with a trait/mixin to avoid the risk of manual errors in what in the end is an already solved problem - if one wants singletons, of course. :) Siemen

Wouldn't a class instance variable be better? I want new singletons for every subclass. http://rmod-pharo-mooc.lille.inria.fr/MOOC/Slides/Week3/ C019-W3S03-Basic-Variables.pdf

I would also have used super new, why BasicNew? But my wish to create subclasses seems to answer that already.

Thanks for your find-grained example, Cyril!

Siemen

On Sat, Jan 21, 2017 at 11:19:24AM +0100, stepharong wrote:

Pay attention that Singleton should not be about access but time.

If you can by adding on instance variable avoid to need a singleton then it means that it was not a singleton.

A singleton is often: you **CANNOT** have two instances at the same time.

Many people do not use well singleton so watch out.

We have several variations of singletons in the Pharo codebase, some of which do allow multiple instances, some of which don't. But they are differentiated by names. https://www.peteruhnak.com/blog/2015/12/06/singleton-variations/ Peter

Stef

On Thu, 19 Jan 2017 11:23:21 +0100, Siemen Baader <siemenbaader@gmail.com> wrote:

...

Is there a trait to quickly create singletons in Pharo, along the way of the Ruby Singleton mixin?

https://ruby-doc.org/stdlib-1.9.3/libdoc/singleton/rdoc/Singleton.html

(and ideally also observer/observable, which I am going to need next..)

-- Siemen

-- Using Opera's mail client: http://www.opera.com/mail/

Yes I like your blog entry. But my point is orthogonal :)

On Sat, Jan 21, 2017 at 11:19:24AM +0100, stepharong wrote:

...

Pay attention that Singleton should not be about access but time.

If you can by adding on instance variable avoid to need a singleton then it means that it was not a singleton.

A singleton is often: you **CANNOT** have two instances at the same time.

Many people do not use well singleton so watch out.

We have several variations of singletons in the Pharo codebase, some of which do allow multiple instances, some of which don't. But they are differentiated by names.

https://www.peteruhnak.com/blog/2015/12/06/singleton-variations/

Peter

...

Stef

On Thu, 19 Jan 2017 11:23:21 +0100, Siemen Baader <siemenbaader@gmail.com> wrote:

...

Is there a trait to quickly create singletons in Pharo, along the way of the Ruby Singleton mixin?

https://ruby-doc.org/stdlib-1.9.3/libdoc/singleton/rdoc/Singleton.html

(and ideally also observer/observable, which I am going to need next..)

-- Siemen

-- Using Opera's mail client: http://www.opera.com/mail/

-- Using Opera's mail client: http://www.opera.com/mail/