I've been working on a tricky problem in Optique (my CLI parser library): how do you make one option's value affect another option's validation and shell completion?

Think git -C <path> branch --delete <TAB>—the branch completions should come from the repo at <path>, not the current directory.

I think I've found a solution that fits naturally with Optique's architecture: declare dependencies between value parsers, then topologically sort them at parse time.

const cwdString = dependency(string());

const parser = object({
  cwd: optional(option("-C", cwdString)),
  branches: multiple(argument(
    cwdString.derive({
      metavar: "BRANCH",
      factory: dir => gitBranch({ dir }),
      defaultValue: () => process.cwd(),
    })
  )),
});

Details in the issue:

https://github.com/dahlia/optique/issues/74#issuecomment-3738381049