From: Logan Capaldo <logancapaldo@...>
Date: 2006-08-01T11:43:32+09:00
Subject: Re: I thought this was the one that worked?


On Jul 31, 2006, at 10:18 PM, Chad Perrin wrote:

> On Tue, Aug 01, 2006 at 09:57:03AM +0900, Logan Capaldo wrote:
>> On Jul 31, 2006, at 7:07 PM, Chad Perrin wrote:
>>
>>> Who said it's different?  I'm just asking about the foregoing
>>> statement
>>> that all blocks are closures (which strikes me as improbable).   
>>> That's
>>> like saying that this will generate a closure (which it won't):
>>>
>>> sub foo {
>>>  return sub { my $bar = 1; print ++$bar };
>>> }
>>>
>>
>> In ruby it does. I'm having difficulty thinking of a simple example
>> to demonstrate it, but basically it grabs the whole environment
>> whether theres a reference to a variable or not. It does this to
>> allow things like:
>>
>> def foo
>>   x = 1
>>   lambda { puts eval("X".downcase) }
>> end
>>
>> You might say, so it just notices that you're using eval. But
>> consider this:
>>
>> def bar
>>   x = 1
>>   lambda { puts send("lave".reverse, "y".sub(/y/, 'x')) }
>> end
>>
>> Obviously contrived examples, but this is part of the reason blocks
>> "leak" memory.
>
> Um, no . . . that's not the equivalent of what I just posted.  To make
> it equivalent, you'd have to do something like this:
>
> def foo
>   lambda { x = 1; puts eval("X".downcase) }
> end
>


It's still a closure.

Like I said it's hard to give you an example, but ruby creates a  
closure for a block regardless. That lambda still has a reference to  
the enclosing scope of foo, even if their are no variables there. Ok  
I came up with a good one:

% cat closure2.rb
class A
   def initialize
     @a = 1
   end

   def foo
     lambda { puts @a }
   end

   def set_var(val)
     @a = val
   end
end

a = A.new
baz = a.foo
baz.call

a.set_var( "Hello" )

baz.call

% ruby closure2.rb
1
Hello


In this case it happens to be closing over "self", but since this is  
ruby there is _always_ a self, all blocks are closures. (and you  
always have the associated overhead)


> -- 
> CCD CopyWrite Chad Perrin [ http://ccd.apotheon.org ]
> print substr("Just another Perl hacker", 0, -2);
>