Property vs Behavior inheritance

John J Barton johnjbarton at
Fri Dec 16 07:34:43 PST 2011

On Fri, Dec 16, 2011 at 4:22 AM, Axel Rauschmayer <axel at> wrote:

> ...
> How you work with object exemplars is almost like with classes: You have
> to be aware which objects are exemplars and which ones are instances. A
> useful sanity check could be to require an exemplar to have a constructor()
> method.
I'll just re-state this paragraph to illustrate that the glass is half

Unlike classes, object exemplars are just objects. There is no language
support for detecting a object exemplar; developers have no help in
avoiding pernicious debilitating bugs caused by data values  in base

> Note that there are always two steps involved in subtyping:
> 1. Extend the shared properties.
> 2. Extend the instance properties.
> 99% of the time this is not what developers want or need.  They want to
extend the shared functions, not the shared properties.

The <| operation solves the same problem that seflish solves: behavior
> inheritance; they both use object literals (optional in selfish); neither
> address the data-property specification problem nor the data-property
> initialization problem.
> Yes, you have to know that instance properties are always added in the
> constructor. But that rule is the same with most class declaration
> proposals that exist. I don’t find that rule problematic, it’s just not how
> things are commonly done in mainstream languages – which tend to have a
> more declarative syntax for instance members.

And I agree that the extra syntax is not good, but the result is very good.
Is there another way? That is the challenge I am posing here.

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