Creating custom minor mode with keybindings

Question

I've written my first custom elisp function for dealing with the output from grep.

(defun open-at-grep ()
  (interactive)
  (let ((thisline (buffer-substring-no-properties (line-beginning-position) (line-end-position))))
    (find-file (nth 0 (split-string thisline ":")))
    (goto-line (string-to-number (nth 1 (split-string thisline ":"))))
    ))

This works, and I can hotkey it with a global key-binding. What I would like to do is to create a minor mode to autoload the keybinding (eventually just getting hooked for files with a custom extension). What I've tried is

(define-minor-mode grepout-mode
  "tool for making use of output of grep -n"
  nil
  "grepout-mode"
  (("C->" .  open-at-grep)))

which throws an error when emacs launches

Invalid function: (C-> . open-at-grep)

The stack-trace from launching emacs --debug-init is not particularly more helpful. I've tried (([C->] . 'open-at-grep)), and other permutations of quotes and not quotes. What am I missing?

Answer 1

Check the documentation for define-minor-mode. It is fairly long, so I will only quote the most relevant section:

:keymap MAP Keymap bound to the mode keymap.  Defaults to MODE-map.
        If non-nil, it should be an unquoted variable name (whose value
        is a keymap), or an expression that returns either a keymap or
        a list of (KEY . BINDING) pairs where KEY and BINDING are
        suitable for define-key.  If you supply a KEYMAP argument
        that is not a symbol, this macro defines the variable MODE-map
        and gives it the value that KEYMAP specifies.

Note well what it says. It expects either a variable name, or an expression that evaluates to a keymap, or a list of pairs. You are trying to use a list of pairs, but lists need to be quoted or else they will be evaluated as code. The quote goes in front of the form to be quoted:

(define-minor-mode grepout-mode
  "tool for making use of output of grep -n"
  nil
  "grepout-mode"
  '(("C->" .  open-at-grep)))

Of course, none of this is specific to defining a minor mode. It is a basic feature of the Emacs Lisp language. Perhaps you should review An Introduction to Programming in Emacs Lisp. To open it inside of Emacs, type C-h i to open the Info viewer. This should start you at the directory of available manuals, otherwise hit d to jump to it. m will let you type in the name of a menu item, or you can search with C-s, etc.

The Emacs Lisp Intro may not go into enough depth to answer all of your questions about quoting and evaluation, so I also recommend reading chapter 10.3 Quoting of the Emacs Lisp manual.