From: Marnen Laibow-Koser <marnen@...>
Date: 2009-11-18T05:47:28+09:00
Subject: Re: Class inside a Method Body

Robert Klemme wrote:
> On 17.11.2009 19:40, Marnen Laibow-Koser wrote:
>> Mike Stephens wrote:
>>> My concern here is when you use libraries. When your user just wants to 
>>> get a list of product categories from the web server, you seem to risk 
>>> Ruby parsing the whole portfolio of library capabilities despite the 
>>> fact you emphatically won't use 99% of them on that transaction.
>> 
>> Not at all.  This is what require is for.  If you don't require a file, 
>> it won't be read.
> 
> Mike's point is, that if you require a library which requires other
> files in turn you might end up loading 50 files although you just need 3
> of them.  That overhead can be significant for short lived programs.

True.

> 
> He has a valid point: basically his question is, how do I declare file
> dependencies in complex libraries without immediately "executing" them
> (means: reading all those files)?  A good approach here is to use
> autoload alone or a combination of autoload and require as I have done
> in the Muppet Laboratories project (see link below).
> 
>>> Autoload does look as though it addresses this.
>> 
>> Autoload just automates require AFAIK.
> 
> Yes, but this is crucial here: autoload will require a file once a
> constant is accessed the first time.  And you can even nest it, i.e. use
> it inside a module.  That way, if a class is never used at all, you
> won't read that file.

With a better explanation of the library example, now I understand what 
the OP was getting at.  You're right.

> 
> You can find an example in my project:
> http://github.com/rklemme/muppet-laboratories/blob/master/lib/animal.rb
> 
>>> The other solution you mentioned was you put require inside a method and 
>>> then Ruby won't execute it until control passes to the method, so if you 
>>> don't need that method, you don't incur the cost of interpreting it. 
>>> Presumably you can't add classes this way 
>> 
>> Of course you can!  Why do you think you can't?
> 
> Exactly.  The required file can contain anything - from zero to multiple
> classes.
> 
>>> and, from a glance at The Ruby 
>>> Programming Language (Flanagan & Matsumoto), there is a risk the 
>>> included code gets treated as belonging to a different scope to the 
>>> calling method (although that might be a plus point).
>> 
>> If you wrap it in a class or module, you get to specify the scope.
> 
> Hmm...  I am not aware of any scoping issues right now but you could
> avoid them by anchoring the class name:
> 
> class ::Foo
> end
> 
> That class is always defined in the root namespace.
> 

Indeed.  I'd be surprised if this were actually necessary, though.

> Kind regards
> 
>   robert


Best,
--
Marnen Laibow-Koser
http://www.marnen.org
marnen@marnen.org
-- 
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