From: Keunwoo Lee (klee@cs.washington.edu)
Date: Tue Nov 05 2002 - 15:05:42 PST
On Sun, 3 Nov 2002, Andrei Alexandrescu wrote:
> In essence AOP is an AST transformer. It allows you to transform the AST
> of classes and functions in well-defined ways: the compiler parses a
> class into its AST, and then the so-called aspect weaver effects
> transformations on certain nodes of that trees (notably function calls).
>
> I was thinking of what relationship AOP has with parametric
> polymorphism, and here's how it feels. Parametric polymorphism can be
> seen as an AST transformation as well. The simplest transformation is
> replacing a type parameter with an actual type name, but this activity
> can lead to quite subtle transformations when you think that that type
> has member functions, nested types, and so on.
A few comments:
+ Almost everything is an AST transformer. For example, the formal
operational semantics of a language is generally specified as a set of
rewrite rules over abstract syntax---this is true of the lambda calculus
and its derivatives, Featherweight Java, and most concurrent calculi.
+ The AspectJ people lately have begun to move the way they view their
language towards a more dynamic semantics, and away from static syntax
tree rewrites. In their latest papers, the way they talk about join
points and so on is in terms of dynamic behaviors rather than static
augmentation of methods etc. The ArchJava example you posted demonstrates
this with its inspection of the runtime call chain. In this dynamic
flavor of AOP, it is a bit weird for aspects to change the type of
something, because types are static (rather than runtime) entities.
Aside from this, however, it seems that AOP potentially describes a
promiscuous intermingling of many concepts: syntax rewrites, type
modification, and altered dynamic semantics. In the face of this fact,
the insistence some AOP advocates that AOP is "not metaprogramming" is a
little confusing. If this is not metaprogramming, what is?
And if AOP is metaprogramming, then its power is essentially limited only
by the expressiveness of the particular metalinguistic constructs you
provide. In a sufficiently expressive AOP system, you could express
*every* language extension as an aspect. And, indeed, that's what you can
do in Lisp, where every program is an s-expression and you can execute any
computable function over a program.
Therefore, the question is not: "How expressive is AOP?" Rather, one
should ask: "How expressive do you want AOP to be, and what constructs
give that expressiveness while retaining modular reasoning, etc.?" My
opinion of AspectJ is that it seems to give you too much power (or the
wrong kind) and not enough modular reasoning.
~k
_______________________________________________
Cecil mailing list
Cecil@cs.washington.edu
http://majordomo.cs.washington.edu/mailman/listinfo/cecil
This archive was generated by hypermail 2.1.5 : Tue Nov 05 2002 - 15:05:48 PST