From: Luke Kanies <luke@...>
Date: 2006-08-02T07:22:33+09:00
Subject: Re: Strategies for autoloading ruby files

Brian Palmer wrote:
> You can use `Module.const_missing` to autoload a file the first time the 
> class in that file is mentioned in the code. So you could write an 
> `Object.const_missing(name)` method that translates the constant into a 
> file name of an expected format, then load that file on-the-fly.

Ironically, I use almost no constants in my code.  Parent classes get 
constants, but subclasses are almost always created dynamically[1], like so:

   Puppet::Type.newtype(:user) do
     ...
   end

Then I always refer to that class by the name 'user' and never by the 
constant.  This is partially because it's easier for me, but also 
because I've written a custom language (no, not a DSL[2]) for Puppet, 
and most of those names are exposed in that language.  E.g., you create 
a 'user' type like above, and users can create user elements in Puppet:

   user { luke:
     comment => "Luke Kanies",
     uid => 100,
     shell => bash,
     ensure => exists
   }

Puppet supports lots of package types (e.g., dpkg, apt, rpm, 
darwinports, gems) and service types (init, smf, etc.); I specify the 
name when I create the class, and users can select those types by name, 
either in the language or directly in Ruby if they're directly accessing 
the library.

In other words, my problem is not how to hook into the autoload, it's 
the guts behind it.

> Amusingly, this is exactly the code example that's given in the Pickaxe 
> version 2 under Module.const_missing, along with a note that the code is 
> in very poor style and is a "perverse kind of autoload functionality".

1 - I do assign constants to them, but only because class inspection 
during error printing did not work consistently unless there's a 
constant assigned to the class.  I just tried reproducing this but could 
not; I don't remember the details well enough, I guess.

2 - See the FAQ, http://reductivelabs.com/projects/puppet/faq.html, the 
fourth question.

-- 
An expert is a person who has made all the mistakes that can be made
in a very narrow field.       - Niels Bohr
---------------------------------------------------------------------
Luke Kanies | http://reductivelabs.com | http://madstop.com