Discussion: Feature appetite

[xxxserxxx]

You mention that you'd accept a PR for combined boolean short flags (-a -b ~= -ab). What is your appetite for these features:

• Argument type assertions. E.g., if the default value is time.Time, the value is returned as a time.Time). Generics may keep this addition small (LOC)
• Choices: if the default value is an array, then only values from the array are accepted as valid. E.g.

  infoCommand.AddArg("subjects...", ["chemistry", "physics", "algorithms")
  command, err := registry.Parse([]string{"--subjects", "poetry"})
  // err => InvalidChoiceError{Arg: "subjects", Value: "poetry"}

• Mandatory/optional. Mandatory flags must be provided, or result in a parsing error. The most backwards-compatible API change would be adding a function, AddReqFlag().

My priority preference are in that order: combined short bool flags, types, and choices. Mandatory flags, I'm not strongly opinionated about, as it's easily enough handled outside the library. But I think the first three -- with Go's new support for generics -- would be relatively minor and easily backwards-compatible.

If you're open to PRs, I'll have a crack at all of them; if you have an opinion on how they look, I'd rather discuss up front than have to change the design at merge time. The biggest impact would be a dependency on Go 1.19+.

Combined short flags is straightforward, API-wise. Type assertions and choices, I think can be added without changing the current API at all.