Okay, so I did understand what you are saying.
Like I said, that's a rather large layering violation, since the function has nothing to do with the expansions happening on its arguments. They are expanded before they ever reach the function. It's essentially allowing functions to modify the syntax around them.
How exactly would this treat e.g.
yt $URL/somefile
Now I couldn't be sure how my arguments would be interpreted, or I'd have to go checking for everything I invoke.
I use youtube-dl myself quite often, and what I do is just type
youtube-dl '
Additionally, it's looking like 3.0 will remove the `?` glob, which is the thing that is most annoying with URLs (though "&" is also a thing).
> I would like to be able to write 'm foobar' and then press tab to get a vertical completion-list. When the user selects a completion-list-item I would like to pass the result-path to the cd command.
I would assume the reason you failed is that the argument you want doesn't match the argument the user typed. Because fish's completions undergo filtering (with some fuzzy-matching, so e.g. `f_c_i` matches `fish_config_interactive`). Otherwise every completion script would have to implement that by itself, and consistently to not confuse people.
For completions, that's the right thing to do.
What you want, I'd argue, isn's a completion. What you want is a selection menu. Which could be implemented with fish's completion pager (we have an open issue about making it accessible to the outside), but isn't the same thing.
JSON here is a red herring, since you don't just want to add items (with descriptions), but also control the behavior of the pager ("orientation" and "on_single_result"). That you could do just with arguments to this `select` command (`list_of_items | select --orientation=vertical --on-single=choose`).