Dynamic configuration

In addition to rebar.config and reltool.config, you can make use of dynamic configuration
based on file:script/2.

If rebar.config.script or reltool.config.script exist, the script file
will be evaluated and the result used as configuration. It also works for custom
rebar.config files. That means rebar -C special_config will check for
existence of special_config.script.

For convenience two bindings (variables) are available in a script during evaluation:

1. CONFIG - result of file:consult/1 if the script file being evaluated also exists
   without the .script extension. Otherwise, [].

2. SCRIPT - filename of the script being evaluated

Simple Example

If you are building fairly complex systems, going to github to fetch deps each time will slow down your development cycle. Serving dependencies locally may be a much faster route, but you don't want to modify the rebar.config and suffer merge conflicts as a result.

The following rebar.config.script file can be kept centrally, and linked into your application directory:

case os:getenv("REBAR_DEPS") of
    false -> CONFIG; % env var not defined
    []    -> CONFIG; % env var set to empty string
    Dir ->
	lists:keystore(deps_dir, 1, CONFIG, {deps_dir, Dir})
end.

Whenever you want to build 'properly' (which you should, regularly), simply call unset REBAR_DEPS (or equivalent), and perform a clean build.

Note that file:script/2 differs from file:consult/1 in that only the result of the last expression is returned. You must therefore take care to return a list of config items. Before that, you may do any form of IO, checking OS environment variables, reading files (perhaps calling file:script/2 on other files) or writing files. As in file:eval/2, each expression is terminated with a full stop.