From: ptkwt@... (Phil Tomson)
Date: 2004-08-04T05:21:32+09:00
Subject: Re: Closure question

In article <25820D5115DEEB48AC514130CA7BA29355F85A@MSMBWHQ05.northamerica.cerner.net>,
Fitzgerald,Greg <GFITZGERALD@CERNER.COM> wrote:
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>I'm trying to make a function act as a counter, but I can't seem to get
>it to work.  Here's what I've tried:
>
>def count
>  counter =3D 1
>  def count
>    proc {counter +=3D 1}.call
>  end
>  counter
>end
>
>
>I was hoping for this:
>count =3D> 1
>count =3D> 2
>count =3D> 3
>
>Instead, I got this:
>count =3D> 1
>count =3D> NoMethodError: undefined method '+' for nil:NilClass
>
>Isn't the idea of a closure that everything in the block is executed in
>the same scope it was defined?  It seems that the name 'counter' is
>referenced under the name 'count', and so when I redefine 'count', the
>'counter' variable gets lost.  Anyone know what's really happening here?
>

The problem is that you've defined a new scope with the definition of the 
inner 'count' method, so the 'counter' variable you're referring to inside 
of the proc is not the same as the one defined in the outer count method.
Also, everytime you call 'count', doesn't the 'counter' variable get 
reinitialized to '1'?  And, it's not clear to me how the inner 'count' 
method would ever get called.  I believe you're seeing the error because 
both the inner and the outer 'count' methods are both called 'count'.

I modified your code somewhat to remove the name collision:
 
irb(main):064:0> def foo
irb(main):065:1>   counter = 1
irb(main):066:1>   def bar
irb(main):067:2>     proc {counter += 1}.call
irb(main):068:2>   end
irb(main):069:1>   bar   #-> note this added call to bar
irb(main):070:1>   counter
irb(main):071:1> end
=> nil
irb(main):072:0> foo
NoMethodError: undefined method `+' for nil:NilClass
	from (irb):67:in `bar'
	from (irb):67:in `call'
	from (irb):67:in `bar'
	from (irb):69:in `foo'
	from (irb):72
	from :0

.... so the inner method, bar, knows nothing about the 'counter' variable 
defined in the outer scope.



Here's how I'd do a counter, but it doesn't involve closures, so maybe it 
doesn't address your question:

class Counter
  def initialize
    @counter = 1
  end
  
  def count
    @counter += 1
  end
end

cc = Counter.new
cc.count #=> 2
cc.count #=> 3

Phil