There was some talk on #nixos about this:

• https://logs.nix.samueldr.com/nixos/2018-08-02#1533176306-1533175472;

The gist of the discussion boils down to:

• -p can be used -p hello bash busybox.
• --keep-var needs to be repeated --keep-var foo --keep-var bar --keep-var baz.

I am personally fine with it being repeated, but I was wondering whether it is fine since there is -p which acts that way.

I would, though, need to add a bit more complexity to the parsing, as right now left is strictly used for -p, so interleaving packages with parameters is possible nix-shell hello bash -p works.

Update (as I was writing). Since nix-shell hello bash -p works, this would be a breaking change. I don't think that this is a good idea to switch to accumulating here unless breaking -p to not read before its use is desired to allow other accumulating parameters in the future.