From: Phil Tomson <rubyfan@...>
Date: 2006-08-29T03:53:17+09:00
Subject: Re: Need - macros feature

On 8/28/06, Logan Capaldo <logancapaldo@gmail.com> wrote:
>
> On Aug 28, 2006, at 1:11 PM, Henry Savr wrote:
>
> > Matthew Johnson wrote:
> >>> Thank you, guys
> >>> Although it's not what I wanted, I have a lot of ideas in some
> >>> directions. It's good for new in Ruby.
> >>> Henry
> >>
> >> Can you describe some of your ideas?  That would certainly help
> >> foster discussion...
> >>
> >> Matthew
> >
> > There is nothiing new. It's as old as Univac
> > What I want is a classic Macroprocessor tool.
> > I mean:
> > I keep pieces of souce code in some place.
> > They may be included in source code as is. More interesting, you can
> > adjust macroprocessor's output (hence, the input for Ruby interpeter)
> > using paremters.
> >
> > E X A M P L E:
> > ===============
> > You wrote the macro once and put it in macro library
> > (here I "invented" the "macrolanguage" on-fly, while writing the
> > example. I  hope, the "language" is clear)
> >
> > ====this is a macro %var with argument %%x definition==============
> > macro: %var (%%x)
> >
> > {def %%x=(value)
> >       @%%x = value
> >       return @%%x
> > end}
> > ======== end of macro %var definition =============================
> > You may consider the macro definition as a template.
> >
> >
> > Now, somewhere in your code you write:
> > ...
> > %var(my_lovely_var)
> > ...
> >
> > Once found the %var, macroprocessor should make substitutions and
> > produce this piece:
> >
> > def my_lovely_var=(value)
> >       @my_lovely_var = value
> >       return @my_lovely_var
> > end
> >
> > which will be inserted into code on the place, where the statement
> > %var(my_lovely_var) was instead of it.
> >
> >
> >
> > Ruby style allows to expect, that it may be done on-fly.
> >
> > So, finally the Ruby interpreter will execute the macroprocessor's
> > output, not input. Besides the macroprocessor's input and output are
> > saved, so you can return to them and change the simple lines of
> > get/set-code to something more complicated and valueable.
> >
> > Again the get/set operations are just an EXAMPLE, used because
> > everyone
> > is familiar with this piece of code. Sure, using attributes is kind of
> > solution for this particular case. (BTW. Thank you, Farrel Lifson, you
> > post convinced me to learn more about attributes on this very
> > beginning
> > stage, and that was very usefull). I want the macro power options
> > themselves.
> >
> I think maybe you underestimate what can be done with meta
> programming in ruby that (almost) obviates the need for macros.
>
> class Module
>    # "macro" definition
>    def var(symbol)
>      define_method("#{symbol}=") { |value| instance_variable_set("@#
> {symbol}", value) }
>    end
> end
>
> class A
>    var :my_lovely_var
> end
> a = A.new
> a.my_lovely_var = 1
>
> Maybe if you can come up with a less trivial example, you can stump
> us, but I doubt it.
>

While Ruby's metaprogramming facilities are indeed powerful, they
still don't allow you to change the syntax of the language as Lisp
macros allow.  So, for example, if I wanted to define a new operator
:= for assignment semantics I can't do that with metaprogramming, but
if Ruby had Lisp-like macros it would be possible.


Phil