Add operator support

[brettlangdon]

Idea revitalized from #9

Originally I wanted to support operators like +, -, / and *, but cut it out of the original scope and now am considering adding them back in.

The simple use case here is for numbers.

primary {
  num_workers = 2;
}
secondary {
  num_workers = 4;
}

total_workers: primary.num_workers + secondary.num_workers;

one_hour = 3600;

primary {
  interval = 5 * one_hour;
}

As well, concatenation for strings would be helpful:

prefix = "prefix";

primary {
    string_setting = prefix + "primary";
}

secondary {
  string_setting = prefix + "secondary";
}

Where is starts to get trickier is with sections, do we merge them together?

primary {
  some_setting: "value";
}

secondary {
  some_setting: "another value";
}

all_settings: primary + secondary;

What is the value of all_settings here?

Also, should consider this for lists when they are supported. Lists will probably be treated as sets and also support unions, intersections and etc

[lode]

I would say this belongs in the programming language using the configuration file, and not in the configuration file itself. A bit like the mustache mantra of "Logic-less templates". Keep it simple.
What's your stance in this?

[brettlangdon]

Yeah, I've gone back and forth on whether it is worth adding or not. Mostly wanted to put up these issues to see what people say.

[lode]

I do see the value in string concatenation (as in the referenced HOCON issue). Especially for i18n config that might be very handy. And number 'concatenation' seems not far away then, but calculating feels a bit off track for a config.

[brettlangdon]

Yeah, I am not 100% on the use case, just figured if I was going to do integer addition, wouldn't be too hard to add the others.