Traits library

Tom Van Cutsem tomvc at
Tue Feb 16 19:18:48 PST 2010

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.


> 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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