Add a package from nixpkgs to your current fish shell

with.fish

function with --description 'import all arguments as nixpkgs args and put their /bin on PATH'
  set arg ( \
    printf '
      let pkgs = import <nixpkgs> {};
      in pkgs.symlinkJoin {
        name = "extra-fish-path";
        paths = with pkgs; [ %s ];
      }
    ' \
    "$argv" \
  )
  set tmpdir (mktemp -d)
  set outLink "$tmpdir/fish-with-nix-gcroot"
  # remove gc root on shell exit
  trap "rm -r $tmpdir" EXIT
  set outPath ( \
    nix-build \
      --out-link "$outLink" \
      -E "$arg" \
  )

  set --prepend PATH "$outPath/bin"
end