How should a function passed to _arguments for zsh shell completion look like?

Question

In my completion function, I'd like to add an option which possible arguments depend on which files are on the system. I have no troubles getting that list of possible arguments, but I have no idea what I should do with it.

This guide talks about FUNCTION as possible action for _arguments. (For some reason I didn't find it in the official docs) And that works well, the function specified is indeed called. But what should I now do with that list? I tried returning it and printing it. In both cases nothing happened.

_files manages to do it somehow, but from looking at the source code I didn't find anything useful. I probably have to store it in some variable, but where?

My code so far:

#compdef _command command

function _command {
    local line

    _arguments -C \
        '(-)'{-h,--help}'[Show help]' \
        '(-u --update -p --platform)'{-u,--update}'[Update]' \
        '(-p --platform -u --update)'{-p,--platform}'[Platform]:whatshouldiputhere:_get_all_platforms' \
        "*::arg:->args"
}

function _get_all_platforms {
    # I know how to get the list of possible completions but I have no idea what to do with it
}

Answer 1

But what should I now do with that list?

You should pass it as arguments to the compadd builtin command:

This builtin command can be used to add matches directly …