From: "T. Onoma" <transami@...>
Date: 2003-11-24T17:35:46+09:00
Subject: Controlled block variables (was: open-uri patch, added progress_proc hook)

In considering some of the things we've been discussing related to the 
open-uri hook, duck_typing, singletons, and block scope.... I have to throw 
in evals, and how they work. I tend to use eval quite often it seems, at 
least in my more interesting "ruby research" coding.

One of the annoyances that I frequently run into is the need to pull variables 
from the current scope into the eval block. Obviosuly to do this one has to 
pass eval a string wilth "#{x}" substituions in it, or concatinations of 
strings,  "..." + x + "...". In fact you HAVE to do the later if the eval 
will have its only substitions. Ie. "..." + ' "#{c}" ' + "...".  But this 
also causes whole literals to be placed into the evalutaion. Eg.

  a = [1,2,3]
  eval "[#{a.join(',')}]"

What would really make eval much more convenient is if it had some type of 
scope "holes" cutting through to the current scope. Of course this would 
require a special meta-syntax, but it should be possible. In fact, I imagine 
we could jsut use the current string substitution for this, since # 
represents a remark anyway. #{ would be a special token. For example:

  x = 'hello'
  eval { x = 1; p "#{x}" }  # => "1"       (as expected)
  eval { x = 1; p #{x} }  # => "hello"      (rabbit hole)

Would something like this be possible --eval with closure? Even if a different 
syntax were needed, this would be a great advantage.

-t0