From: ThoML <micathom@...>
Date: 2008-02-08T23:14:59+09:00
Subject: Re: Symbol#to_proc, lazy evaluation (was Re: idiom I've not seen 	before)

> Io (like Lisp, I gather) allows lazy evaluation of method arguments.

Are you referring to Lisp's macro facility? AFAIK macros are usually
expanded at load/compile time. Since everything is a s-expression in
lisp, the macro gets its arguments as list and can process them. The
macro call is then replaced with the result. Or something along this
line.

It seems Io provides a similar facility to treat code as data, which
is cool. Unfortunately I'm not able to compile it. IIRC Forth was able
to do something similar too BTW (just a side-note).

Since ruby doesn't have this feature (unless you use parsetree and
ruby2ruby maybe), the ruby equivalent are strings, I'd say. One could
thus write something like:

    class Array
        def select_by(snippet)
            rv = []
            self.each do |e|
                rv << e if e.instance_eval '%s %s' % [e.inspect,
snippet]
            end
            rv
        end
    end

    a = [1,2,3,4,5]
    a.select_by '> 3'

Which is about the same as your io example. It would be nice of course
to be able to have real macros that are expanded at load time so that
one could write something like the following which is incorrect ruby
syntax and would have to be rewritten before the parser tries to make
sense out of it:

    module Macros
        def select_by(code)
            %{select {|e| e #{code}}}
        end
    end

    a = [1,2,3,4,5]
    a.select_by(> 3)

This would require that macro names are unique though. But it's just a
delirious day-dreaming anyway.

Regards,
Thomas.