From: Dumaiu <Dymaio@...>
Date: 2006-07-23T05:15:08+09:00
Subject: Re: *_eval

Erik Veenstra wrote:
> Can somebody explain me, in detail, the differences between
> aModule.instance_eval, aModule.module_eval and
> aModule.class_eval?
>
> I'm especially interested in the differences in context.
>
...

	I think inspect() and object_id() confuse the issue, and trying to use
instance_eval() at module scope is about as much fun as putting a
screwdriver through your eye.  My own views are as follows:
module_eval() is intended for defining methods dynamically, and its
receiver should be of class Module.  Given

	module M; end
	mod = M # or some other module

I would use

	m.module_eval {
		def foo; end
	}

as a faster, cleaner way of accomplishing

	eval <<-EOS
		module #{m.to_s}
			def foo; end
		end
	EOS

which is hampered by its reliance on a string eval().
	On the other hand, I see instance_eval() as primarily useful for
*calling* a blockful of methods, with a dynamically-determined
receiver.  Say you have a bunch of stuff you want some object to
do--e.g., you know it'll be Enumerable, so you have some Enumerable
methods--

	module Enumerable
		def func()
			any? whatever
			sort
			to_a
		end
	end

	enum.func()

but then you realize that you might want to use this more generally,
not just on Enumerable objects.  So you wrap it:

	func = proc do
		any? whatever
		sort
		to_a
	end

and when you want somebody to call it, use

	obj.instance_eval &func

The alternative would be

	func = proc do |receiver|
		receiver.any? whatever
		receiver.sort
		receiver.to_a
	end

	func.call(obj)

which is a less elegant factorization.

...
>
> I'll switch back to good old Lisp...
>
> Good bye, brave Ruby people...
> 
> It was a nice time...
> 
> So long...
> 

     Bye.