Traits library

[Kam Kasravi]

Hi Tom

thanks for your excellent clarifications below. I agree traits is a way to compose different blocks from different 
'classes' which would avoid the limitations of single-inheritance. One use case when using traits in this way is 
whether traits (either using Trait.create or Object.create) could be used to replace prototypical inheritance rather than 
classical inheritance. That is, can traits be used to compose different blocks from different instances rather than
different classes.

kam



________________________________
From: Tom Van Cutsem <tomvc at google.com>
To: Kam Kasravi <kamkasravi at yahoo.com>
Cc: es-discuss <es-discuss at mozilla.org>
Sent: Wed, February 17, 2010 2:49:43 PM
Subject: Re: Traits library


Hi Kam,

Thanks for the clarification. As far as I can tell, what you are describing is an example that would require trait composition to be postponed until runtime (as it depends on runtime context). The way I see it, the traits library would not prevent that: since traits are not static entities but just property maps that are manipulated at runtime, I see no fundamental obstacle to writing a generic dispatching function that, given some context, returns the appropriate traits or trait composition making up a drawing editor.

However, I would like to point out that if this is your desired use case, then traits may not be the right building block. As you point out yourself, the example you're describing seems like a perfect match for multi-methods. I do not think that 'dynamic trait composition' is a good alternative for multi-methods. At least, that's not the use case for which traits have been designed. Traits are an alternative to composition by inheritance. As is the case in typical single/mixin/multiple-inheritance schemes, the relationship between the inheriting entities is usually known (and declared) statically and is immutable. I see traits more like a static building block that can be used to organize code (while enabling a higher reuse potential than that provided by classes+single-inheritance).

Some details:

It doesn't sound like traits would 
>enable this 'pattern' given different methods need to be mixed in
> with 
>appropriate context from the implementation and all methods are final. 

Methods are 'final' upon trait composition only when a property map is instantiated using "Trait.create". If "Object.create" is used, the instantiated object is a plain Javascript object with all of the malleability this implies.
 
>Passing attributes like 
>background, border in the property descriptor would seem to be an abuse 
>of the meta-data framework by passing data not meta-data.

I completely agree. When I proposed the use of attributes to store data in property descriptors in my earlier message, I thought you meant to store meta-data.

Cheers,
Tom
 

>
>thx
>kam
>
>
>
________________________________
From: Tom Van Cutsem <tomvc at google.com>
>To: Kam Kasravi
> <kamkasravi at yahoo.com>
>Cc: es-discuss <es-discuss at mozilla.org>
>Sent: Tue, February 16, 2010 7:18:48 PM
>Subject: Re: Traits library
>
>
>>Hi Kam,
>
>
>If I understand the implementation of traits, it provides a ES5 compatible way of composing 'final' properties to an existing object's prototype.
>>Options provide the meta-data required to define the property descriptor such as required, etc. Do traits provide an ability to bind 
>>a 'context' to the property in the form of a closure so that the property may be provided with additional information?
>>Effectively a way to curry or export additional information required by the trait when it is called in the context of the object it was added to.
>
>
>I'm not entirely sure I understand the question. Do you have a particular use case in mind or can you give an example to clarify things?
>As far as I can understand, if what you want is additional meta-data stored in property descriptors, this can be done by adding additional attributes to the descriptors. In effect, that's what the library already does for 'required' and 'conflicting' properties.
>
>
>Cheers,
>Tom
> 
>
>>
>>thx
>>kam
>>
>>
>>
>>
>>>>
________________________________
From: Tom Van Cutsem <tomvc at google.com>
>>To: es-discuss <es-discuss at mozilla.org>
>>Sent: Tue, February 16, 2010 2:55:50 PM
>>Subject: Traits library
>>
>>
>>>>Hi,
>>
>>
>>Mark Miller and I have been working on a small traits library for Javascript. It is documented here: <http://code.google.com/p/es-lab/wiki/Traits>
>>
>>
>>Traits are reusable building blocks for classes, very similar to mixins, but with less gotchas. For example, traits support explicit conflict resolution upon name clashes and the order in which traits are composed is not significant.
>>
>>
>>In a nutshell:
>>- The library is designed for ES5, but backwards-compatible with existing ES3 implementations.
>>- Our library represents traits as ES5 property maps (objects mapping property names to property descriptors). The library exports:
>> - a convenient trait "constructor" to generate property maps from object literals.
>> - a number of "trait combinators" to compose property maps.
>> - a function that can "instantiate" such property maps into objects (analogous to the ES5 Object.create function, but with awareness about trait-specific property semantics).
>>
>>
>>The interesting thing about our choice of transparently representing traits as ES5 property maps is that our library can be used as a general-purpose library for manipulating property descriptors in addition to its use as a library to compose and instantiate traits.
>>
>>
>>A small expository example that uses the library:
>><http://code.google.com/p/es-lab/source/browse/trunk/src/traits/examples.js>
>>
>>
>>Mark and I were both surprised at how well Javascript accommodates a trait library with very little boilerplate. However, there is one catch to implementing traits as a library. Traits, like classes, are normally simply declared in the program text, but need not necessarily have a runtime representation. Trait composition is normally performed entirely at compile-time (in trait lingo this is called "flattening" the traits). At runtime, no trace of trait composition is left.
>>
>>
>>Because we use a library approach, traits are not declarative entities and must have a runtime representation. Thus, there is a runtime overhead associated with trait creation and composition. Moreover, because the implementation is oblivious to traits, multiple objects instantiated from the same trait "declaration" don't share structure. However, we did design the library such that, if traits are specified using object literals and property renaming depends only on string literals (which is the common case), a partial evaluator could in principle perform all trait composition statically, and replace calls to Trait.create with a specialized implementation that does support structural sharing between instances (just like an implementation that notices multiple calls to Object.create with the same property descriptor map can in principle arrange for the created objects to share structure).
>>
>>
>>Any feedback on our design is welcomed. In particular, it'd be interesting to hear how hard/easy it would be for an implementation to recognize the operations performed by our library in order to perform them statically.
>>
>>
>>Cheers,
>>Tom
>
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