From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2009-12-24T20:19:09+09:00
Subject: Re: loses scope?

On Thu, Dec 24, 2009 at 6:59 AM, Jeff Shantz <rubyforum@jeffshantz.com> wrote:
> Jesús Gabriel y Galán wrote:
>> irb(main):001:0> class A
>> irb(main):002:1> end
>> => nil
>> irb(main):005:0> a = 3
>> => 3
>> irb(main):006:0> A.class_eval do
>> irb(main):007:1* define_method(:go) do
>> irb(main):008:2* puts a
>> irb(main):009:2> end
>> irb(main):010:1> end
>> => #<Proc:0xb7dbffd0@(irb):7>
>> irb(main):011:0> A.new.go
>> 3
>> => nil
>
> Just curious... is there any difference between using define_method and
> def in this case?  It seems they achieve the same effect.  Why would one
> be preferred over the other?
>
>>> A = Class.new
> => A
>>> A.class_eval do
> ?>     define_method(:test) do
> ?>       puts "Testing!"
>>>     end
>>>   end
> => #<Proc:0xb72bdc40@(irb):3>
>>> A.new.test
> Testing!
> => nil
>>> B = Class.new
> => B
>>> B.class_eval do
> ?>     def test
>>>       puts "Testing!!"
>>>     end
>>>   end
> => nil
>>> B.new.test
> Testing!!
> => nil
>

The difference is that def starts a new scope, it's not a closure, so
it doesn't see its surrounding scope. In the following case (as well
as the original case by the OP), the local variable 'a' is seen in the
block passed to define_method, but not to the body of the 'def'
keyword, since it's not a closure:

irb(main):001:0> class A; end
=> nil
irb(main):002:0> a = 3
=> 3
irb(main):003:0> A.class_eval do
irb(main):004:1* define_method(:go) do
irb(main):005:2* puts a          # this a is the a outside the class_eval
irb(main):006:2> end
irb(main):007:1> end
=> #<Proc:0xb788d530@(irb):4>
irb(main):008:0> A.new.go
3
=> nil
irb(main):009:0> A.class_eval do
irb(main):010:1* def no_go
irb(main):011:2> puts a      #this one is undefined
irb(main):012:2> end
irb(main):013:1> end
=> nil
irb(main):014:0> A.new.no_go
NameError: undefined local variable or method `a' for #<A:0xb7870584>
	from (irb):11:in `no_go'
	from (irb):14
	from :0

Hope this helps,

Jesus.