From: Wilbert Berendsen <wilbert@...>
Date: 2003-01-27T00:15:38+09:00
Subject: cascading configuration variables

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Hi,

a little follow-up on loading config data as Ruby source files. I want my 
project to load some full system defaults from a file, and then some 
user-specified overrides from another file, and finally some special 
overrides that are set on the commandline or from the calling environment.

I would like the variables with their values to be accessible as constants, 
because I think that is faster than hash lookups, and it looks elegant, 
because they will get referenced often from other script snippets that get 
loaded after the config is read.

I figure that I can use modules to carry constants. Eg.

module Conf
	include load ("userdefaults.conf", true)
	include load ("systemdefaults.conf", true)
end

# (This is done once, when starting the program.)
# Now the Conf module contains all constants from systemdefaults,
# but overridden by userdefaults (they are not overwritten by the system
# defaults, because constants are not changed by include when they are set.

# nou create a new class to hold new specified settings
# (This can occur often during a session)

confc = Class.new ( OurBaseClass )  # OurBaseClass contains methods etc.
confc.const_set "CFLAGS", "our value"  # specific setting
confc.const_set "VAR2", 1234
confc.class_eval "include Conf"

object = confc.new

# our object now has all constants defined as needed, and all the methods from
# OurBaseClass.

# The constants will be referenced to like in my previous thread, by little
# snippets of Ruby code loaded on demand.

My question: is this the most elegant (and fastest) way to do this? Or should 
I use hashes where usersettings can simply overwrite system settings. The 
code snippets then can't just to:
	ALSA && depend "audio/alsa-driver"
but must look like:
	uses["ALSA"] && depend "audio/alsa-driver"
or, using a method,
	use "ALSA" && depend "audio/alsa-driver"

The 1st one really looks best to me.

Esp. wat is bothering me a little is that I need to create a new class for 
every new object that I want to spawn that needs possibly different settings 
for a few constants (user- and sys-defaults remain the same)

So what could be the most efficient way to set up cascading config options 
like this?

thanks for all your help and thoughts!
Wilbert

- -- 
Wilbert Berendsen (http://www.xs4all.nl/~wbsoft/)
To understand recursion, one must first understand recursion.
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