Writing GNU Emacs Extensions

Setting Up the Mode

There are two circumstances under which a user expects to enter Crossword mode. One is when visiting a file that contains a crossword grid from an earlier session. Another is when creating a brand-new crossword grid.

Creating a brand-new crossword grid requires creating an empty buffer and filling it in using crossword-insert-grid. The act of entering a major mode shouldn't change buffers or alter a buffer's contents, so crossword-mode will only be for entering Crossword mode in a buffer already containing a crossword grid. We'll devise a separate command, crossword, for creating a grid from scratch.

Here's a start at defining crossword:

(defun crossword (size)
  "Create a new buffer with an empty crossword grid."
  (interactive "nGrid size: ")
  (let* ((grid (make-crossword size))
         (buffer (generate-new-buffer "*Crossword*")))
    (switch-to-buffer buffer)
    (crossword-insert-grid grid)
    (crossword-place-cursor 0 0)      ;start in upper-left corner
    ...))

We'll leave this function unfinished for now, but before we move on, let's note some interesting things about this function:

1. (interactive "nGrid size: "). The letter n is one of a few code letters for interactive that instruct Emacs to prompt the user for a value. These letters allow you to specify a prompt string, as we've done here. This interactive declaration means, "Prompt the user with the string "Grid size: ", and read a number in response."

   What if this command took two arguments, a number and, say, a string? What would the ...