Re: [Pharo-users] singleton trait

[stepharong]

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
 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



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Re: [Pharo-users] singleton trait

[Sven Van Caekenberghe]

> On 21 Jan 2017, at 11:59, Peter Uhnak  wrote:
> 
> 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/

That's a good summary, thanks Peter.

> Peter
> 
>> 
>> Stef
>> 
>> On Thu, 19 Jan 2017 11:23:21 +0100, 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
>> 
>> 
>> 
>> -- 
>> Using Opera's mail client: http://www.opera.com/mail/
> 

Re: [Pharo-users] singleton trait

[Peter Uhnak]

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
>  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/

Re: [Pharo-users] singleton trait

[stepharong]

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.

Stef

On Thu, 19 Jan 2017 11:23:21 +0100, 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




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

Re: [Pharo-users] singleton trait

[Siemen Baader]

On Thu, Jan 19, 2017 at 3:51 PM, Siemen Baader 
wrote:

>
> On Thu, Jan 19, 2017 at 11:33 AM, Cyril Ferlicot D. <
> cyril.ferli...@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
>

Re: [Pharo-users] singleton trait

[Cyril Ferlicot D.]

On 19/01/2017 15:51, Siemen Baader wrote:
> 
> 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 use a class variable because I try to avoid a maximum the use of the
Singleton pattern. Most of the time it is wrongly used and it make
things harder to maintain in a long time. So I do not want a whole
hierarchy of Singleton :) (For some reasons about why Singletons are
evil you can easily find articles on internet. I looked quickly and
found this for example:
https://blogs.msdn.microsoft.com/scottdensmore/2004/05/25/why-singletons-are-evil/)

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

When I do Singletons I change the new method to raise an error but I let
the developer use #basicNew in order to not forbid him to create a
second class if he knows what he's doing. With that we can test the
singleton with another instance that the one the application use.

> Thanks for your find-grained example, Cyril!
> 

You're welcome :)

> Siemen


-- 
Cyril Ferlicot

http://www.synectique.eu

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



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Re: [Pharo-users] singleton trait

[jtuc...@objektfabrik.de]

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-Variables.pdf


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.

Re: [Pharo-users] singleton trait

[Siemen Baader]

On Thu, Jan 19, 2017 at 11:33 AM, Cyril Ferlicot D. <
cyril.ferli...@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 ]
>

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

Re: [Pharo-users] singleton trait

[Siemen Baader]

On Thu, Jan 19, 2017 at 11:37 AM, Sven Van Caekenberghe 
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. 
> 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
> >
>
>
>

Re: [Pharo-users] singleton trait

[Denis Kudriashov]

2017-01-19 11:37 GMT+01:00 Sven Van Caekenberghe :

> 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).
>

We could have statefull traits in future

Re: [Pharo-users] singleton trait

[Sven Van Caekenberghe]

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.  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
> 

Re: [Pharo-users] singleton trait

[Cyril Ferlicot D.]

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



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[Pharo-users] singleton trait

[Siemen Baader]

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