Allow optional values (not simply optional args with value notation)

[patcon]

From my reading of the docs, this isn't possible, but feel free to correct me!

The distinction being that an optional argument using value notation is currently possible

-b --bump=[type] 'bump the version point automatically (options: patch, minor, major)'

Which would allow all of:

~$ mycommand --some-arg --bump=patch
~$ mycommand --some-arg --bump=major
~$ mycommand --some-arg # doesn't bump version

But I don't think there's a way to allow an optional value, like so:

~$ mycommand --some-arg --bump=patch
~$ mycommand --some-arg --bump # could be same result as above
~$ mycommand --some-arg # doesn't bump version

Any help or guidance is appreciated! If this is already possible, I'm happy to help update the docs to better convey this distinction :)

[kbknapp]

Try setting Arg::min_values(0). This should work, but I also can't remember if I special cased 0 values...

Let me know if that doesn't work 😉

[patcon]

Thanks so so much bunch, @kbknapp! :)

EDIT: haha "so so much bunch"...

[nodakai]

It would be great if a brief note about this use case could be added to Arg::min_values().

Also, in this use case the difference between ArgMatches::value_of() returning Some and ArgMatches::is_present() returning true is important, and I think this part of the documentation of value_of() should be revised.

If the option wasn't present at runtime it returns None.

Because it seem to suggest that it returns None iff the option wasn't present.

if matches.is_present("bump") {
  if let Some(v) = matches.value_of("bump") {
    println!("bump == {:?}", v);
  } else {
    println!("bump"); // matters only under `min_values(0)`
  }
} else {
  println!("do not bump");
}

[kbknapp]

@nodakai I'd be good adding that blurb or updating the docs if you could open a new issue for it. Could you point out a link to the sections which should be made more clear? I have the problem of guilty knowledge and it's hard to for me to see which parts aren't exactly clear since I wrote the library and am intimately familiar with it 😜