Provide an option/argument defaulting mechanism

[rvesse]

As seen in #49 there is a general need to support additional ways for users to supply default values to Airline based CLIs. Another common use case from my own $dayjob usage is taking defaults from environment variables. Currently doing this requires providing custom application specific solutions. It would be nice if Airline provided a standard plugin point for this that could then be customised by end users.

My rough idea is to add a new internal phase to the parsing process that would occur after all options and arguments have been passed BUT before we enforce restrictions final validation. The default value supplied would be injected into the list of parsed options/arguments as appropriate ONLY when that particular option/argument hadn't been supplied. Normal restrictions on that option/argument should still apply as usual.

The new DefaultsProvider interface would be pluggable allowing users to configure their desired one on their parser configuration. Likely we'd supply some basic implementations for configuration files (#49) and environment variables.

[gsmet]

Hey @rvesse ,

Still using Airline in my Quarkus GitHub App lib and very happy with it. I had a request tangential to this issue as I've been hit by the same issue over and over: would you accept a PR making @Arguments an empty list instead of null when we don't have any?

It's a bit unpractical to have to special case null every time.

I suppose it would be easy enough to get this part into 3.x? Might break compatibility though. Let me know and I might be able to provide a PR to fix it.

[rvesse]

Yes I don't see any problem with a PR that does that

It seems like it would only break compatibility is if people are relying on the null to not do something and I don't think we've ever explicitly promised that behaviour?

Usually if you don't want to have to deal with null you can explicitly default the field to an empty list e.g.

@Arguments
private List<String> args = new ArrayList();

Then if Airline doesn't find any arguments it'll leave that field alone and you'll have the default empty list in it

[rvesse]

Also I think you can have a partial list and Airline will simply add to it e.g.

@Arguments
private List<String> args = new ArrayList(Arrays.asList("foo"));

NB If you use an immutable list there e.g. private List<String> args = List.of("foo") then I think it'll fail, but can't remember if it does or not TBH