Descriptions from the class side

I know all the advantages of having the descriptions defined in the
instance side. But I find one drawback.

Let's suppose I want to implement a tool that works at meta level, and
I want a list of the available descriptions of a class in order to
build a query where each description will map to a column.

Is there any way I can obtain all the instance side descriptors
without having to instantiate my object?


MyModel has
#descriptionA
#descriptionB
#descriptionC

I want something like: MyModel>>#magritteDescriptionOfInstances and
get the above mentioned descriptions.

I know some descriptions will need specific instance variables in
order to work, I would like also to categorize/filter those
descriptions that are "class side friendly" (it is, they don't require
instance specific behavior).

Otherwise I don't see other option than to partition the descriptors
to both instance and class side.

E.g.:
MyModel>>#descriptionA
  <magritteDescription>
  ^self class descriptionA


MyModel class>>#descriptionA
  <magritteDescription>
  ^MAStringDescription new...

Any clues?


Esteban A. Maringolo
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Errata:
Where it says: MyModel>>#magritteDescriptionOfInstances
It should say:  MyModel class>>#magritteDescriptionOfInstances

:)
Esteban A. Maringolo


2014-02-27 19:46 GMT-03:00 Esteban A. Maringolo <[hidden email]>: _______________________________________________
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Hi Esteban,

I “solved” this problem by creating a template object. This template is created once, for all classes and when I need a "description of the class”, I ask the for description of the template.

But you are right: most descriptions are static and could be defined on the class side. Allowing them to be defined on both class side and instance side might be a good idea. And this would allow you to ask for the description on the class side as well. And when you ask the description of a concrete object, it get’s enriched with the instance side descriptions.

Diego

On 27 Feb 2014, at 23:47, Esteban A. Maringolo <[hidden email]> wrote:


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2014-02-28 4:55 GMT-03:00 Diego Lont <[hidden email]>:
> Hi Esteban,
>
> I "solved" this problem by creating a template object. This template is created once, for all classes and when I need a "description of the class", I ask the for description of the template.

I guess you mean one template instance per class/hierarchy.
Do you have a class instance variable for this template instance?


> But you are right: most descriptions are static and could be defined on the class side. Allowing them to be defined on both class side and instance side might be a good idea. And this would allow you to ask for the description on the class side as well. And when you ask the description of a concrete object, it get's enriched with the instance side descriptions.


Exactly.

Dolphin Smalltalk has its own meta descriptions (named
"AspectDescriptors" or simply "aspects") that provide specific
inspectors and other nice features for its inspectors and toolset.

On Object's instance side the #publishedAspects is implemented as:

Object>>#publishedAspects
"Answer a <LookupTable> of the <Aspect>s published by the receiver."

| aspects |
aspects := self class publishedAspectsOfInstances.
...
^aspects

And in the class side:
Object class>>#publishedAspectsOfInstances
"Answer a LookupTable of AspectDescriptors that describe the aspects published
    by an instance of the receiver. Overridden by subclasses to add the aspects
    published locally."

^(LookupTable new)
add: (self newInstanceAspect: #yourself class: Aspect);
yourself


CompiledCode class>>#publishedAspectsOfInstances
"Answer a <LookupTable> of the <Aspect>s published by instances of the
receiver."

^(super publishedAspectsOfInstances)
add: (Aspect name: #methodClass) beReadOnly;
add: (Aspect name: #header) beReadOnly;
add: (Aspect integer: #argumentCount) beReadOnly;
add: (Aspect multilineString: #getSource) beReadOnly;
add: (Aspect name: #byteCodes) beReadOnly;
add: (Aspect multilineString: #disassembly) beReadOnly;
add: (Aspect name: #literals);
yourself

You normally work at the instance side, but almost everything is
"defined" at the class side. So you can know what is and what is not
available.
Of course subclasses can "remove" some descriptors from the
LookupTable (optimized Dictionary). Although in Magritte that is
managed by the MAContainer.


Regards!

--
Esteban!

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