From: Joel VanderWerf <vjoel@...>
Date: 2004-05-16T17:13:30+09:00
Subject: Unique Object for Each source file


This is a silly little bit of code, but it's sort of an answer to 
http://ruby-talk.org/99775, "RCR: Unique Object for Each source file".

Also, I had been thinking about proposing an RCR to let Kernel#load 
return the last value in the file it loads, so it would be easy to 
construct an object in a script file and hand it back to the main 
program. But after reading back to the thread at 
http://ruby-talk.org/62660, that didn't seem like the best way to go. 
So, based on Nobu's suggestion in http://ruby-talk.org/62727, here is a 
Script class which does the following:

- program.rb:

     require 'script'
     script = Script.load("script.rb")
     p script::VALUE
     script.run

- script.rb:

     VALUE = [1,2,3]
     def run
       puts "#{self} running."
     end

- output:

     $ ruby program.rb
     [1, 2, 3]
     #<Script:/tmp/script.rb> running.

The value returned by Script.load is a module, and you can access its 
constants and module methods, as expected. You can, of course, define 
any number of constants or methods in the scope of the script file. 
Defining things at the top level in your script file behaves as you 
expect, like definitions at the top level of ruby itself, but in the 
scope of the script. More precisely, every "def foo" results in 
'module_function "foo"' getting invoked, so you can call foo on the 
script object itself, back in the main program. This also means that 
instance variables defined at the top level of your script are 
accessible from the module functions that you define at the top level.

Script has implementations of #load and #require that, called from 
within a "script", try to find files locally (relative to the script 
file) and then fall back to the Kernel versions of #load and #require, 
which look in $LOAD_PATH. If a local file is found, its definitions go 
into the same scope as the original script. The intent is that if you 
have main script file that loads some others, and everything works 
nicely with "ruby script.rb", then it should also work with 
Script.load("script.rb"), except that all of your definitions will be 
wrapped in the module returned by Script.load. (And it will not break if 
you Dir.chdir.)

Also, it is possible to pass "arguments" to a script by attaching a 
block to Script.load and using it to define constants in the script's scope.

The implementation of Script is very simple and is included below. A 
complete package with documentation and examples is at 
http://redshift.sourceforge.net/script.

It's seems like too trivial a thing to put on RAA, especially with a 
grand name like "script", so I'll just leave where it is for now, unless 
someone has a better suggestion.

-- script.rb
class Script < Module
   attr_reader :main_file
   attr_reader :dir
   attr_reader :loaded_features

   class << self
     alias load new
   end

   def initialize(main_file)
     @main_file = File.expand_path(main_file)
     @dir = File.dirname(@main_file)
     @loaded_features = {}

     yield self if block_given?
     load_in_module(main_file)
   end

   def load(file, wrap = false)
     load_in_module(File.join(@dir, file))
     true
   rescue MissingFile
     super
   end

   def require(feature)
     unless @loaded_features[feature]
       @loaded_features[feature] = true
       file = File.join(@dir, feature)
       file += ".rb" unless /\.rb$/ =~ file
       load_in_module(file)
     end
   rescue MissingFile
     @loaded_features[feature] = false
     super
   end

   class MissingFile < LoadError; end

   def load_in_module(file)
     module_eval(IO.read(file), file)
   rescue Errno::ENOENT => e
     if /#{file}$/ =~ e.message
       raise MissingFile, e.message
     else
       raise
     end
   end

   def method_added(name)
     module_function(name)
   end

   def to_s
     "#<#{self.class}:#{File.join(dir, File.basename(main_file))}>"
   end
end