From: "Paolo \"Nusco\" Perrotta" <paolo.nusco.perrotta@...>
Date: 2007-12-19T00:04:59+09:00
Subject: Re: Difference between alias and alias_method

On Dec 17, 11:12 pm, "jwmerr...@gmail.com" <jwmerr...@gmail.com>
wrote:
> Don't know exactly why, but wrapping alias in a class_eval seems to
> fix the problem:
>
> class_eval("alias old_initialize initialize")
>
> Using ri, I couldn't even find who exactly alias belongs to (i.e. is
> it defined in Kernel, Object, Module, or what, and is it a class
> method of one of these?)  Hopefully someone else can weigh in and
> clear this up.

This is because "alias" is not a method - it's a keyword (that trips
me up every time I put a comma between the old and new method name,
and the compiler complains).

You can use "alias" in two contexts. You normally use it in a class
definition to alias an instance method of the class. It's not enough
that the class is "self" - you really must be inside the class
definition. That's why this works:

  MyClass.class_eval{ alias :to_string :to_s }

while this doesn't (at least, not as you might think):

  MyClass.instance_eval{ alias :to_string :to_s }

What happens if you use alias *outside* a class definition instead?
When you do that, you're actually aliasing a method of "self" (the
current object) to a singleton method:

 irb(main):001:0> c = Object.new
 => #<Object:0x2e8f0d8>
 irb(main):002:0> c.instance_eval { alias :clazz :class }
 => nil
 irb(main):003:0> c.clazz
 => Object

(If you don't know what a "singleton method" is, you can find many
threads about the topic on this group. Be advised that this will
require you to take a journey into Ruby metaprogramming).

As a special case of the above, if "self" is a class, then you're
aliasing an existing method to a singleton method of the class - and a
singleton method of the class is what we usually call a class method.
That's why instance_eval() might surprise you:

 irb(main):003:0> String.instance_methods[0..1]
 => ["%", "select"]
 irb(main):004:0> String.instance_eval { alias :x :instance_methods }
 => nil
 irb(main):006:0> String.x[0..1]
 => ["%", "select"]

Bottom line: if you want "alias" to behave as you probably want it to
behave, do like Jason suggested and wrap it in a class_eval(). :)

  Paolo Perrotta
  Bologna, Italy