From: "David A. Black" <dblack@...>
Date: 2004-08-24T11:15:26+09:00
Subject: Re: *with* block?

Hi --

On Tue, 24 Aug 2004, Daniel Sheppard wrote:

> If you're just after configuring the instance variables, you might go
> for:
> 
> my_obj = boxWidget.new
> my_obj.instance_eval do
>   @x = 0
>   @y = 10
>   @width = 100
>   @height = 100
> end
> 
> Though doing so will make you code fairly fragile - setting variables in

sub(/fairly/,"extremely") :-) There's absolutely no guarantee of a
one-to-one correspondence between instance variables and setter/getter
methods -- and the way those methods are implemented could even change
from one release of a library to another.  

> this way will bypass the setters, so you can't customize your setting
> actions. (I was suprised that instance_eval didn't actually grant the
> ability to call methods on the object without prefixing them)

It's because it's a parse-time thing: all the bare identifiers on the
left of ='s are designated local variables.  So #instance_eval is in
the same boat as other contexts (instance methods, etc.).  Also, its
block acts as a closure:

  a = 1
  "".instance_eval { a = 2 }  # pointless, but just to illustrate
  p a  # 2


David

-- 
David A. Black
dblack@wobblini.net