Functions which are mode dependent

[Heime]

I need some help understanding how to automatically enable some functionality
when loading a file with some major mode.

For instance I have a function that sets 


(defun expr-elisp ()

  (setq imenu-generic-expression
    `( ("defun" ,(concat "^\\s-*"
                   "(\\(defun\\|define-minor-mode\\)\\s-+"
                   "\\(\\(\\sw\\|\\s_\\)+\\)") 2) )) )


(defun expr-selector ()

  (cond
    ((eq major-mode 'emacs-lisp-mode)  (expr-elisp))
    (t  (expr-generic)) ))

I have put the code above in a minor mode. 

;;;###autoload
(define-minor-mode tema-minor-mode

  (if tema-minor-mode
        (expr-selector)
      (message "Tema Deactivated")))

with hooks to load the minor mode automatically for specific major modes.

(add-hook emacs-lisp-mode-hook #'tema-minor-mode)