Taxonomy Of Object Composition

As I wrote the last two entries, "Enhanced EJB" and "Fallacies of Inheritance", I noticed some parallels, that is the notion of acquisition, inheritance and delegation seem related.

So out of curiosity, I consulted the all wise google with a query "acquisition semantics prototype", hoping that it'll answer my question on the relationship between prototype based languages (i.e. Self, JavaScript) with the acquisition semantics found in Zope.

To my surprise, I encountered this research performed by the same people doing work on fluid AOP (i.e. CaesarJ). It turns out that they've already thought this out in detail and have created a taxonomy of the different "aspects" of object composition:

• Overriding - The ability of the modification to override methods defined in the base. In M(B), M's definitions hide B's definitions with the same name. Self-invocations within B ignore redefinitions in M.
• Transparent redirection - The ability to transparently redirect Bs this to denote M(B) within the composition.
• Acquisition - The ability to use definitions in B as if these were local methods in M(B) (transparent forwarding of services from M to B).
• Subtyping - The promise that M(B) fulfills the contract specified by B, or that M(B) can be used everywhere B is expected.
• Polymorphism - The ability to (dynamically or statically) apply M to any subtype of B.

	inheritance	object composition	delegation	mixin inheritance
overriding	x	-	x	x
redirection	x	-	x	x
acquisition	x	-	x	x
subtyping	x	-	x	x
polymorphic	-	dynamically	dynamically	statically

Note: Delegation refers to prototype oriented inheritance.

They've written a compiler based on Kopi (GPL) that makes it easier to express different object composition semantics. Semantics that you possibly never knew existed. This just looks like a wonderful testbed to explore the pros and cons of composing object using varying kinds of semantics.

It could help answer the question of which composition mechanism is best for a particular domain. For example, is the Zope acquisition semantics, the very one that drives this blog, the ideal mechanism for composing Content Management Systems? Could there be a better alternative?

So head over there to download the thing, it's called "latte macchiato" and it'll keep you awake late at night exploring!