conditional :mode

[gdkrmr]

I would like to use a mode only under certain conditions for a filetype:

(use-package ess-site
  :mode (("\\.R\\" . R-mode)
         (unless ess-enable-julia ("\\.R\\" . ess-julia-mode))))

[skangas]

Would something like this work?

(use-package ess-site
  :preface
  (defun my/R-or-julia-mode ()
    (if ess-enable-julia
        (ess-julia-mode)
      (R-mode)))
  :mode (("\\.R\\" . my/R-or-julia--mode)))