From: Hal Fulton <hal9000@...>
Date: 2004-04-05T08:04:29+09:00
Subject: Re: Variable names

David King Landrith wrote:
> After 2 days and several emails, I still have no idea how the characters 
> are getting evaluated and resolved by the interpreter and at what point 
> these resolved or evaluated characters are disposed of in favor of the 
> objects that they represent.  Moreover, I still have no idea if or why 
> this question may reflect a category mistake.

OK, let me take a stab at this.

If I understand you correctly, you want to retrieve the original
expression on which a method was called.

In the current Ruby implementation, this is absolutely impossible.
Ruby does not at this time expose such a deep and detailed knowledge of
the state of the interpreter's internals.

It is true that the expression is represented as a string of characters,
and that this eventually corresponds to an object on which you may call
a method.

But the resolution or evaluation as you call it happens so early, so
deep in the bowels of the interpreter, that the running code cannot get
access to the original text.

Ruby is a very introspective language. But no language (in my
repertoire) has such a deep, deep introspection -- not Java, not Ruby,
not anything else (that I know).

Lisp is probably an exception -- in fact, I would think it would be.
And there are likewise probably others.

The Ruby interpreter is a black box. Exposing its internals to the
running program requires a great deal of work on the part of the
language implementor. What's more, it makes sense to hide everything
that doesn't NEED to be exposed, for the sake of modularity.

So there are only certain things that are exposed. eval(), for instance,
is like a little window into the guts of the interpreter. But it is a
window that took man-months to get right.

 > I have no experience in writing compilers/interpreters, and I don't
 > believe that I should have to in order to learn about such topics.

If you want to solve a problem of such gigantic complexity, and there
has been no provision for this kind of thing in advance, then yes,
you do need knowledge of writing interpreters, because you will have
to write one of your own that does what you want.

In short: The Ruby interpreter doesn't do what you want.

Your alternatives are:
1. Use a different language, such as Lisp.
2. Write your own Ruby interpreter.
3. Persuade Matz to invest a few man-months in this endeavor.

Does that help any?

Hal