[Feature request] Native zsh completion

Picocli currently can create bash completion files, and zsh has a compatibility tool to read bash completion files.

However, this only exposes basic autocompletion features (what bash itself support), and does not take advantage of the features zsh offer.

For instance, zsh can show a description for each command/option, as well as option aliases (short vs long options). There’s a thousands pictures on the internet; here’s one example for the git command:

This behavior cannot be obtained from a bash completion file.

Therefore, I propose that a new Autocomplete.zsh static method be added, to output a zsh-native completion script that includes more details, like command and option description and aliases.

[remkop]

Reference material:

• https://blog.mads-hartmann.com/2017/08/06/writing-zsh-completion-scripts.html
• https://thevaluable.dev/zsh-completion-guide-examples/
• https://github.com/zsh-users/zsh-completions/blob/master/zsh-completions-howto.org

[remkop]

@yschimke I saw your message on #502. Thanks! I am still working on some parser issues, but ZSH completion is high on my list of priorities. Once I make some progress on this your feedback will be very welcome!

UPDATE 2020/11/16

I was thinking about redesigning the logic for generating completion scripts to follow the Visitor pattern; I need to prototype this a bit to see how useful this would be. Something like this:

interface ModelVisitor<R,P> {
  R visit(CommandSpec command, P p);
  R visit(ArgGroupSpec group, P p);
  R visit(OptionSpec option, P p);
  R visit(PositionalParamSpec positional, P p);
}

Current call tree:

bash(String, CommandLine)
  generateEntryPointFunction
    generateFunctionCallsToArrContains
      [generateFunctionCallsToArrContains...] - recursively for nested subcommands
  generateFunctionForCommand - for all subcommands and nested descendant sub-subcmds
    generateCompletionCandidates - options with completion candidates
    generateOptionsSwitch - other options for which completions can be generated
    generatePositionParamCompletionCandidates - positionals with completion candidates
    generatePositionalParamsCases - other positionals for which completions can be generated