Traits library

Tom Van Cutsem tomvc at
Wed Feb 17 14:49:43 PST 2010

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

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


> thx
> kam
> ------------------------------
> *From:* Tom Van Cutsem <tomvc at>
> *To:* Kam Kasravi <kamkasravi at>
> *Cc:* es-discuss <es-discuss at>
> *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>
>> *To:* es-discuss <es-discuss at>
>> *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: <
>> 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:
>> <
>> >
>> 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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