From: Brian Candler <b.candler@...>
Date: 2010-02-16T17:54:47+09:00
Subject: Re: 'def', but with a closure

Albert Schlef wrote:
> BTW, anybody knows what exactly `def` does? Until now I thought it does 
> this:
> 
>    Lookup the enclosing class (or module) and add the method
>    to its m_tbl.
> 
> Based on Hidetoshi's code it seems `def` does this:
> 
>    Look at 'self'. If it's a class (or a module), add the method
>    to its m_tbl. If it isn't, get its singleton and add the method
>    to *its* m_tbl.

I believe it's slightly more complex than that.

In executing code, you're familiar with the idea of "the current 
object". This is made available as 'self'.

However there's also a more hidden concept of "the current class". It's 
rather difficult to get hold of this, but it's where def defines 
instance methods.

"The current class" is set inside a class ... end construct. But as 
you've found, this starts a new scope. So you can use class_eval 
instead, which sets both the current object and the current class.

Compare:

class Foo; end
Foo.instance_eval do
  def foo; puts "XXX!"; end
end
Foo.foo       # you have made a class method

class Bar; end
Bar.class_eval do
  def bar; puts "YYY!"; end
end
Bar.new.bar   # you have made an instance method

Anyway, this is moot if you are working with closures, because 'def' 
also starts a fresh scope.

> It's surprising to see that a few "important" libraries are using eval. 
> ActiveRecord, for example. I wonder why.

'def' methods are more efficient at runtime (and also less liable to 
unforeseen side-effects), precisely because they do not have access to 
outer scopes. If you want to 'def <foo>' where <foo> is dynamic, you 
have to use eval.
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