Original 20-Nov-1994, last mod 24-Nov-1994.

Module Customizations Mechanism

The idea here is to provide a regular interface by which users register custom configurations for packages (module or groups of modules), and packages instantiate those customizations. The customization registation process should not require loading of the package itself, and the instantiation process should be easy and automated, but still under control of the package.

I'd previously given some thought to this issue, and implemented a rough stab at dealing with it. I've posted a prior version, and am including a revamped version, with explanation, as a basis for further dialogue on the subject...

Here is an excerpt from the code docs, describing the mechanism:

Module customizations mechanism.

Regular interface by which users configure custom settings for modules, and
modules implement those customizations at initialization time.

$Id: custs-prop.html 4678 2002-02-28 16:45:09Z barry $
Created by Ken.Manheimer@nist.gov,  16-Nov-1994

The customization process does not depend on prior loading of the target
module in order to register customizations.  The module, conversely, controls
adoption of the customizations.

Users employ the custom.config() routine to specify custom settings.  Modules
use the custom.implement() routine to:
  - register default values for vars, and
  - to deploy the actual value for the var:
      - the custom value, if one has been designated using custom.config(), 
      - or else the default value, as specified in the custom.implement() call.

custom.status() returns info about the status of specific customization vars,
and custom.cancel() remove registration for a variable.`

In order to enable special provisions for custom vars, modules can use
custom.implement() to designate routines that implement the var's actual
assignment.  The FUNCTION must take three values - module, variable name, and
value.  It is responsible for actually assigning the variable, and should
return the value assigned. 

Custom vars with no designated assignment function actually use the 'assign'
routine, below.  It simply assigns the value to the var within the module.

(A few, trivial alternate assignment functions are also provided:
  - 'loudassign', which does same thing as assign, but prints an informative
    announcement about it, and
  - 'ignore', which does not actually do the assignment.)"""