Alchemist is a legacy mode.
(use-package alchemist
:ensure t
:init
(setq alchemist-mix-command "/usr/local/bin/mix")
(setq alchemist-iex-program-name "/usr/local/bin/iex"))
(use-package elixir-mode
:ensure t
:init (add-hook 'elixir-mode-hook
(lambda () (add-hook 'before-save-hook 'elixir-format nil t))))
(provide 'elixir)