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julian

Subclassing Array

I am playing with Amber trying to port a small project that I use when
learning a new language. In this I tried to subclass the Array class
but it does not seem to work. When I inspect an instance of my
subclass it is an Array not my subclass of array. I am not a
Smalltalk expert but I did the same thing in Pharo and it worked as I
would have expected.

Is this a bug or a limitation of the Amber implementation?

Is there a workaround?

Thanks
Julian

Nicolas Petton

Re: Subclassing Array

Hi Julian!

Ahhh, there are a few classes that you shouldn't subclass, unless you
know what you are doing.

Array is one of them. Do not subclass it. It won't work in JavaScript
anyway. There are some funny techniques to try to subclass array in JS,
but they're far from being perfect :)

In general, don't subclass Classes that map to a JavaScript constructor.
See js/boot.js

https://github.com/NicolasPetton/amber/blob/master/js/boot.js#L559

Cheers,
Nico

On Fri, 2012-01-20 at 02:01 -0800, julian wrote:

> I am playing with Amber trying to port a small project that I use when
> learning a new language. In this I tried to subclass the Array class
> but it does not seem to work. When I inspect an instance of my
> subclass it is an Array not my subclass of array. I am not a
> Smalltalk expert but I did the same thing in Pharo and it worked as I
> would have expected.
>
> Is this a bug or a limitation of the Amber implementation?
>
> Is there a workaround?
>
> Thanks
> Julian

julian

Re: Subclassing Array

OK, thanks, I thought it might be something like that. I've come
across problems with subclassing array in JS before - it works OK in
some JS implementations but not others. I had hoped that Amber might
hide that nastiness but I suppose you can't without compromising
efficiency too much.

Julian

On Jan 20, 11:19 am, Nicolas Petton <[hidden email]> wrote:

> Hi Julian!
>
> Ahhh, there are a few classes that you shouldn't subclass, unless you
> know what you are doing.
>
> Array is one of them. Do not subclass it. It won't work in JavaScript
> anyway. There are some funny techniques to try to subclass array in JS,
> but they're far from being perfect :)
>
> In general, don't subclass Classes that map to a JavaScript constructor.
> See js/boot.js
>
> https://github.com/NicolasPetton/amber/blob/master/js/boot.js#L559
>
> Cheers,
> Nico
>
>
>
>
>
>
>
> On Fri, 2012-01-20 at 02:01 -0800, julian wrote:
> > I am playing with Amber trying to port a small project that I use when
> > learning a new language. In this I tried to subclass the Array class
> > but it does not seem to work. When I inspect an instance of my
> > subclass it is an Array not my subclass of array. I am not a
> > Smalltalk expert but I did the same thing in Pharo and it worked as I
> > would have expected.
>
> > Is this a bug or a limitation of the Amber implementation?
>
> > Is there a workaround?
>
> > Thanks
> > Julian

gokr

Re: Subclassing Array

On 01/20/2012 12:25 PM, julian wrote:
> OK, thanks, I thought it might be something like that. I've come
> across problems with subclassing array in JS before - it works OK in
> some JS implementations but not others. I had hoped that Amber might
> hide that nastiness but I suppose you can't without compromising
> efficiency too much.

But you can subclass OrderedCollection though.

regards, Göran

Nicolas Petton

Re: Subclassing Array

On Fri, 2012-01-20 at 12:54 +0100, Göran Krampe wrote:
> On 01/20/2012 12:25 PM, julian wrote:
> > OK, thanks, I thought it might be something like that. I've come
> > across problems with subclassing array in JS before - it works OK in
> > some JS implementations but not others. I had hoped that Amber might
> > hide that nastiness but I suppose you can't without compromising
> > efficiency too much.
>
> But you can subclass OrderedCollection though.

Well...Not for long, I was planning to make OrderedCollection an alias
for Array.

Cheers,
Nico

>
> regards, Göran

Dave Mason

Re: Subclassing Array

On 2012-Jan-20, at 7:23 , Nicolas Petton wrote:

> On Fri, 2012-01-20 at 12:54 +0100, Göran Krampe wrote:
>> But you can subclass OrderedCollection though.
>
> Well...Not for long, I was planning to make OrderedCollection an alias
> for Array.

Will it have at:ifAbsentPut: ? I admit I've been following Amber from afar to date, so its OrderedCollection may not have this, but is is so nice for dynamic programming algorithms!

What about addFirst/removeFirst/addLast/removeLast?

../Dave

Nicolas Petton

Re: Subclassing Array

On Tue, 2012-01-24 at 21:50 -0500, Dave Mason wrote:

> On 2012-Jan-20, at 7:23 , Nicolas Petton wrote:
>
> > On Fri, 2012-01-20 at 12:54 +0100, Göran Krampe wrote:
> >> But you can subclass OrderedCollection though.
> >
> > Well...Not for long, I was planning to make OrderedCollection an alias
> > for Array.
>
> Will it have at:ifAbsentPut: ? I admit I've been following Amber from afar to date, so its OrderedCollection may not have this, but is is so nice for dynamic programming algorithms!
>
> What about addFirst/removeFirst/addLast/removeLast?

I'll check these methods. If there are missing I'll add them.

Cheers,
Nico

>
> ../Dave

Hannes Hirzel

Re: Subclassing Array

In reply to this post by julian

On 1/20/12, julian <[hidden email]> wrote:

Hi Julian

Instead of subclassing array just subclass object and have an instance
variable holding the array. Many people consider this to be better
OO-style -- use delegation instead of subclassing.

Kind regards
Hannes

Hannes Hirzel

Re: Subclassing Array

In reply to this post by Nicolas Petton

On 1/25/12, Nicolas Petton <[hidden email]> wrote:

> On Tue, 2012-01-24 at 21:50 -0500, Dave Mason wrote:
>> On 2012-Jan-20, at 7:23 , Nicolas Petton wrote:
>>
>> > On Fri, 2012-01-20 at 12:54 +0100, Göran Krampe wrote:
>> >> But you can subclass OrderedCollection though.
>> >
>> > Well...Not for long, I was planning to make OrderedCollection an alias
>> > for Array.
>>
>> Will it have at:ifAbsentPut: ? I admit I've been following Amber from
>> afar to date, so its OrderedCollection may not have this, but is is so
>> nice for dynamic programming algorithms!
>>
>> What about addFirst/removeFirst/addLast/removeLast?
>
> I'll check these methods. If there are missing I'll add them.

This easily maps

Smalltalk JavaScript

myArr addFirst: anElement myArr.unshift(anElement);
myArr removeFirst myArr.shift();
myArr addLast: anElement myArr.push(anElement)
myArr removeLast myArr.pop()

This may be easily verified e.g. in Firefox 9 with menu 'Tools' / 'Web
developer' / 'Web console'

Regards
Hannes

> Cheers,
> Nico
>
>>
>> ../Dave
>
>
>

Array-shift-unshift-push-pop.PNG (62K) Download Attachment

julian

Re: Subclassing Array

In reply to this post by Hannes Hirzel

On Jan 25, 2:13 pm, "H. Hirzel" <[hidden email]> wrote:
> Instead of subclassing array just subclass object and have an instance
> variable holding the array. Many people consider this to be better
> OO-style -- use delegation instead of subclassing.

Yes, that's exactly what I have done. I agree over use of inheritance
is not a good thing.

Julian

Hannes Hirzel

Re: Subclassing Array

Yes, this is a very prominent recommendation.

First chapter of the 'gang of four' book:

"Favor 'object composition' over 'class inheritance'." (Gang of Four 1995:20)

http://en.wikipedia.org/wiki/Design_Patterns_(book)

HJH

On 1/26/12, julian <[hidden email]> wrote:

> On Jan 25, 2:13 pm, "H. Hirzel" <[hidden email]> wrote:
>> Instead of subclassing array just subclass object and have an instance
>> variable holding the array. Many people consider this to be better
>> OO-style -- use delegation instead of subclassing.
>
> Yes, that's exactly what I have done. I agree over use of inheritance
> is not a good thing.
>
> Julian
>