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OrderedCollection as an instance variable

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sergio_101

OrderedCollection as an instance variable

I'm implementing an instance variable as an OrderedCollection, and am doing something idiotic.

I have an Artist class.

An Artist can have many tracks:

tracks

^ tracks ifNil: [ self tracks: OrderedCollection new ]

tracks: anObject

tracks := anObject

Doing something like:

a := Artist new.

t := Track new.

a tracks add: t

Gives me the error:

Artist(Object)>>doesNotUnderstand: #add:

Once I get past that error, which will evaluate `a tracks`, I can run this.

I get why it works the second time, but I don't get why it thinks the instance of Artist is the receiver of add: on the first run.

Thanks!

----
peace,
sergio
photographer, journalist, visionary

Public Key: http://bit.ly/29z9fG0
#BitMessage BM-NBaswViL21xqgg9STRJjaJaUoyiNe2dV
http://www.codeandmusic.com
http://www.twitter.com/sergio_101
http://www.facebook.com/sergio101

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Sven Van Caekenberghe-2

Re: OrderedCollection as an instance variable

because your #tracks: returns self, not the collection value

> On 24 Jul 2019, at 16:46, sergio ruiz <[hidden email]> wrote:
>
> I'm implementing an instance variable as an OrderedCollection, and am doing something idiotic.
>
> I have an Artist class.
>
> An Artist can have many tracks:
>
> tracks
> ^ tracks ifNil: [ self tracks: OrderedCollection new ]
>
> tracks: anObject
> tracks := anObject
>
> Doing something like:
>
> a := Artist new.
> t := Track new.
> a tracks add: t
>
> Gives me the error:
>
> Artist(Object)>>doesNotUnderstand: #add:
>
> Once I get past that error, which will evaluate `a tracks`, I can run this.
>
> I get why it works the second time, but I don't get why it thinks the instance of Artist is the receiver of add: on the first run.
>
> Thanks!
> ----
> peace,
> sergio
> photographer, journalist, visionary
>
> Public Key: http://bit.ly/29z9fG0
> #BitMessage BM-NBaswViL21xqgg9STRJjaJaUoyiNe2dV
> http://www.codeandmusic.com
> http://www.twitter.com/sergio_101
> http://www.facebook.com/sergio101
>

sergio_101

Re: OrderedCollection as an instance variable

Oh!

how about this:

tracks: anObject

tracks := anObject.

^ tracks

On Jul 24, 2019, at 10:48 AM, Sven Van Caekenberghe <[hidden email]> wrote:

because your #tracks: returns self, not the collection value

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sergio_101

Re: OrderedCollection as an instance variable

In reply to this post by Sven Van Caekenberghe-2

hmm…

maybe this is cleaner..

tracks

tracks ifNil: [ self tracks: OrderedCollection new ].

^ tracks

because your #tracks: returns self, not the collection value

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Paul DeBruicker

Re: OrderedCollection as an instance variable

In reply to this post by sergio_101

You should change your #tracks method to

tracks
^tracks ifNil:[tracks:=OrderedCollection new]

or

tracks
tracks ifNil:[self initializeTracks]
^tracks

initializeTracks
tracks:=OrderedCollection new.

Then you don't have to worry about the implementation of #tracks: changing
in the future if you want to do something else with/in it.

--
Sent from: http://forum.world.st/Pharo-Smalltalk-Users-f1310670.html

Marcus Denker-4

Re: OrderedCollection as an instance variable

In reply to this post by sergio_101

> On 24 Jul 2019, at 17:30, sergio ruiz <[hidden email]> wrote:
>
> hmm…
>
> maybe this is cleaner..
>
> tracks
> tracks ifNil: [ self tracks: OrderedCollection new ].
> ^ tracks
>

I write these methods as

tracks
^tracks ifNil: [ tracks := OrderedCollection new ].

- one line
- does not mix using accusers and not using accessors.

Marcus

sergio_101

Re: OrderedCollection as an instance variable

tracks
^tracks ifNil: [ tracks := OrderedCollection new ].

- one line
- does not mix using accusers and not using accessors.

Okay, I totally get the subtlety now. I don’t even need an accessor.

Thanks!

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Richard Sargent

Re: OrderedCollection as an instance variable

Administrator

I will add that I prefer the static model to tell the truth about its data types. So, I prefer having an #initialize method which ensures the new object is created in a self-consistent way and anyone looking at it in an inspector will see useful and correct information. Leaving instance variables uninitialized is a excellent way to generate Does Not Understand errors (perhaps not in the short-term, but definitely in the long-term).

On Wed, Jul 24, 2019 at 8:39 AM sergio ruiz <[hidden email]> wrote:

tracks
^tracks ifNil: [ tracks := OrderedCollection new ].

- one line
- does not mix using accusers and not using accessors.

Okay, I totally get the subtlety now. I don’t even need an accessor.

Thanks!

----
peace,
sergio
photographer, journalist, visionary

Public Key: http://bit.ly/29z9fG0
#BitMessage BM-NBaswViL21xqgg9STRJjaJaUoyiNe2dV
http://www.codeandmusic.com
http://www.twitter.com/sergio_101
http://www.facebook.com/sergio101

Esteban A. Maringolo

Re: OrderedCollection as an instance variable

I agree with that, but in some cases, like reading objects from an
ORM, you would be creating lots of instances of objects that are going
to be discarded right away.
This would put no stress on GC since it would happen in the "new"
space, but would consume more memory (although I never measured how
much more).

Esteban A. Maringolo

On Wed, Jul 24, 2019 at 1:14 PM Richard Sargent
<[hidden email]> wrote:

>
> I will add that I prefer the static model to tell the truth about its data types. So, I prefer having an #initialize method which ensures the new object is created in a self-consistent way and anyone looking at it in an inspector will see useful and correct information. Leaving instance variables uninitialized is a excellent way to generate Does Not Understand errors (perhaps not in the short-term, but definitely in the long-term).
>
>
> On Wed, Jul 24, 2019 at 8:39 AM sergio ruiz <[hidden email]> wrote:
>>
>>
>> tracks
>> ^tracks ifNil: [ tracks := OrderedCollection new ].
>>
>> - one line
>> - does not mix using accusers and not using accessors.
>>
>>
>> Okay, I totally get the subtlety now. I don’t even need an accessor.
>>
>> Thanks!
>>
>> ----
>> peace,
>> sergio
>> photographer, journalist, visionary
>>
>> Public Key: http://bit.ly/29z9fG0
>> #BitMessage BM-NBaswViL21xqgg9STRJjaJaUoyiNe2dV
>> http://www.codeandmusic.com
>> http://www.twitter.com/sergio_101
>> http://www.facebook.com/sergio101
>>

Pharo Smalltalk Users mailing list

Re: OrderedCollection as an instance variable

In reply to this post by Marcus Denker-4

A book that looks explores implementations like that and that always had
a place on my desk is this

https://www.amazon.ca/Smalltalk-Style-Suzanne-Skublics/dp/0131655493
I can highly recommend it.

Sebastian

On 2019-07-24 8:32 a.m., Marcus Denker wrote:

>
>> On 24 Jul 2019, at 17:30, sergio ruiz <[hidden email]> wrote:
>>
>> hmm???
>>
>> maybe this is cleaner..
>>
>> tracks
>> tracks ifNil: [ self tracks: OrderedCollection new ].
>> ^ tracks
>>
> I write these methods as
>
>
> tracks
> ^tracks ifNil: [ tracks := OrderedCollection new ].
>
> - one line
> - does not mix using accusers and not using accessors.
>
>
> Marcus
>

Richard O'Keefe

Re: OrderedCollection as an instance variable

In reply to this post by sergio_101

Comment 1.

  You probably meant to write

    tracks: aCollection

      ^tracks := aCollection

Comment 2.

  This is a poor design.  As it is, any object can replace the tracks of an artist

  with *anything*.  And even without doing that, any object can add and remove

  items to an artist's tracks, even if the added items are not Tracks.

  There are a number of OO design principles, notably the Law of Demeter and

  of course the GRASP patterns, which basically say "never mutate another

  object's parts, ask IT to do the mutation."

  I have come to regard the self-encapsulation "pattern" as an anti-pattern.

  This is Smalltalk heresy, admittedly, but it doesn't so much break (real)

  encapsulation as set it on fire, defenestrate it, and urinate on the

  smouldering remains.

So this is the way I'd do it.

   Object subclass: #Artist

     instanceVariableNames: 'tracks'

     methods for: 'initialising'

       initialize

         tracks := OrderedCollection new.

         ^self

     methods for: 'checking'

       invariant

         ^(tracks isMemberOf: OrderedCollection) and: [

           tracks allSatisfy: [:each | each isKindOf: Track]]

     methods for: 'accessing tracks'

       addTrack: aTrack.

         tracks add: aTrack. 

         ^track

       tracksDo: doBlock

         tracks do: doBlock

with no way for outside code to accesss 'tracks' directly.

On Thu, 25 Jul 2019 at 02:47, sergio ruiz <[hidden email]> wrote:

I'm implementing an instance variable as an OrderedCollection, and am doing something idiotic.

I have an Artist class.

An Artist can have many tracks:

tracks
^ tracks ifNil: [ self tracks: OrderedCollection new ]

tracks: anObject
tracks := anObject

Doing something like:

a := Artist new.
t := Track new.
a tracks add: t

Gives me the error:

Artist(Object)>>doesNotUnderstand: #add:

Once I get past that error, which will evaluate `a tracks`, I can run this.

I get why it works the second time, but I don't get why it thinks the instance of Artist is the receiver of add: on the first run.

Thanks!

----
peace,
sergio
photographer, journalist, visionary

Public Key: http://bit.ly/29z9fG0
#BitMessage BM-NBaswViL21xqgg9STRJjaJaUoyiNe2dV
http://www.codeandmusic.com
http://www.twitter.com/sergio_101
http://www.facebook.com/sergio101

Herby Vojčík

Re: OrderedCollection as an instance variable

In reply to this post by sergio_101

On 24. 7. 2019 17:30, sergio ruiz wrote:
> hmm???
>
> maybe this is cleaner..
>
> tracks
> tracks ifNil: [ self tracks: OrderedCollection new ].
> ^ tracks

IMO, return ifNil: value is an understood and used idiom, so I'd say

^ tracks ifNil: [ tracks := ... ]

is the most clean way. Maybe look at sender of ifNil: and see for
yourself which is the most idiomatic way to do lazy getter in the
current image (there will be lots of these usages there).

Herby

>>
>> because your #tracks: returns self, not the collection value
>
> ----
> peace,
> sergio
> photographer, journalist,??visionary
>
> Public Key: http://bit.ly/29z9fG0
> #BitMessage BM-NBaswViL21xqgg9STRJjaJaUoyiNe2dV
> http://www.codeandmusic.com
> http://www.twitter.com/sergio_101
> http://www.facebook.com/sergio101
>

Herby Vojčík

Re: OrderedCollection as an instance variable

In reply to this post by Richard O'Keefe

On 25. 7. 2019 4:27, Richard O'Keefe wrote:

> Comment 2.
> ?? This is a poor design.?? As it is, any object can replace the tracks
> of an artist
> ?? with *anything*.?? And even without doing that, any object can add and
> remove
> ?? items to an artist's tracks, even if the added items are not Tracks.
> ?? There are a number of OO design principles, notably the Law of
> Demeter and
> ?? of course the GRASP patterns, which basically say "never mutate another
> ?? object's parts, ask IT to do the mutation."

As outlined nicely in this classic:
https://alanknightsblog.blogspot.com/2011/10/principles-of-oo-design-or-everything-i.html.