From: Logan Capaldo <logancapaldo@...>
Date: 2006-08-01T11:50:28+09:00
Subject: Re: I thought this was the one that worked?


On Jul 31, 2006, at 10:30 PM, Chad Perrin wrote:

> On Tue, Aug 01, 2006 at 09:50:23AM +0900, Logan Capaldo wrote:
>>
>> On Jul 31, 2006, at 6:48 PM, Chad Perrin wrote:
>>
>>> On Tue, Aug 01, 2006 at 07:05:31AM +0900, Logan Capaldo wrote:
>>>>
>>>> On Jul 31, 2006, at 5:13 PM, Chad Perrin wrote:
>>>>
>>>>> On Tue, Aug 01, 2006 at 04:57:48AM +0900, Logan Capaldo wrote:
>>>>>>
>>>>>> All these examples are lexical scoping. Ruby doesn't really have
>>>>>> dynamic scoping although you can sort of abuse instance
>>>>>> variables to
>>>>>> achieve similar effects.
>>>>>>
>>>>>> The difference is that blocks are closures, where def, class, and
>>>>>> module aren't.
>>>>>
>>>>> Wait . . . you mean that *all blocks* are automagically lexical
>>>>> closures, as though declared lexical variables within them have
>>>>> gone out
>>>>> of scope?  I imagine I'm probably misunderstanding you, but if  
>>>>> not,
>>>>> that's a pretty nifty bit of trickery.
>>>>>
>>>> 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.
>>>
>>
>> Yes. It is just like perl.
>>
>> % cat closure.rb
>> def foo
>>   bar = 1
>>   lambda { puts (bar += 1) }
>> end
>>
>> baz = foo()
>>
>> baz.call
>> baz.call
>>
>> % ruby closure.rb
>> -:13: warning: don't put space before argument parentheses
>> 2
>> 3
>
> Okay.  Looks like a closure.  It looks like a closure because of the
> relationship of bar to the return-value block of code.  I've been told
> that all blocks are closures, though -- and I don't see how it's  
> still a
> closure if the "bar = 1" is removed from foo.
>
> -- 
> CCD CopyWrite Chad Perrin [ http://ccd.apotheon.org ]
> "It's just incredible that a trillion-synapse computer could actually
> spend Saturday afternoon watching a football game." - Marvin Minsky
>

Oh yeah, here's another example:

def foo
   return lambda { puts eval("x") }, binding
end

closure, bnding = foo()
eval("x = 7", bnding)

closure.call


Basically, it's impossible not to create a closure with a block in  
ruby, there is always an implicit closure.