From: Logan Capaldo <logancapaldo@...>
Date: 2006-08-01T09:57:03+09:00
Subject: Re: I thought this was the one that worked?


On Jul 31, 2006, at 7:07 PM, Chad Perrin wrote:

> On Tue, Aug 01, 2006 at 07:54:33AM +0900, Jacob Fugal wrote:
>> On 7/31/06, Chad Perrin <perrin@apotheon.com> wrote:
>>> On Tue, Aug 01, 2006 at 07:05:31AM +0900, Logan Capaldo wrote:
>>>> I'm not sure I understand your question. All blocks (by blocks I  
>>>> mean
>>>> do / end and { } ) are (lexically scoped) closures.
>>>
>>> I'll use a Perl example:
>>>
>>> sub foo {
>>>  my $bar = 1;
>>>  return sub { print ++$bar };
>>> }
>>>
>>> my $baz = foo();
>>>
>>> Voila.  $baz contains a lexical closure.  This is the case  
>>> because the
>>> return value from foo() was lexically "closed" by virtue of $bar  
>>> going
>>> out of scope, but its value still being accessible via the coderef
>>> returned from foo() and assigned to $baz.
>>
>> How is this different from:
>>
>>  def foo
>>    bar = 1
>>    return proc{ bar += 1; puts bar }
>>  end
>>
>>  begin
>>    puts bar
>>  rescue Exception => e
>>    puts e
>>  end
>>  # => undefined local variable or method `bar' for main:Object
>>
>>  baz = foo
>>  baz.call # => 2
>>  baz.call # => 3
>>  baz.call # => 4
>
> 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.



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