Info Catalog elisp: Key Binding Conventions elisp: Tips elisp: Compilation Tips

elisp: Programming Tips

 
 D.3 Emacs Programming Tips
 ==========================
 
 Following these conventions will make your program fit better into Emacs
 when it runs.
 
    • Don’t use ‘next-line’ or ‘previous-line’ in programs; nearly
      always, ‘forward-line’ is more convenient as well as more
      predictable and robust.  Text Lines.
 
    • Don’t call functions that set the mark, unless setting the mark is
      one of the intended features of your program.  The mark is a
      user-level feature, so it is incorrect to change the mark except to
      supply a value for the user’s benefit.  The Mark.
 
      In particular, don’t use any of these functions:
 
         • ‘beginning-of-buffer’, ‘end-of-buffer’
         • ‘replace-string’, ‘replace-regexp’
         • ‘insert-file’, ‘insert-buffer’
 
      If you just want to move point, or replace a certain string, or
      insert a file or buffer’s contents, without any of the other
      features intended for interactive users, you can replace these
      functions with one or two lines of simple Lisp code.
 
    • Use lists rather than vectors, except when there is a particular
      reason to use a vector.  Lisp has more facilities for manipulating
      lists than for vectors, and working with lists is usually more
      convenient.
 
      Vectors are advantageous for tables that are substantial in size
      and are accessed in random order (not searched front to back),
      provided there is no need to insert or delete elements (only lists
      allow that).
 
    • The recommended way to show a message in the echo area is with the
      ‘message’ function, not ‘princ’.  The Echo Area.
 
    • When you encounter an error condition, call the function ‘error’
      (or ‘signal’).  The function ‘error’ does not return.  
      Signaling Errors.
 
      Don’t use ‘message’, ‘throw’, ‘sleep-for’, or ‘beep’ to report
      errors.
 
    • An error message should start with a capital letter but should not
      end with a period.
 
    • A question asked in the minibuffer with ‘yes-or-no-p’ or ‘y-or-n-p’
      should start with a capital letter and end with ‘? ’.
 
    • When you mention a default value in a minibuffer prompt, put it and
      the word ‘default’ inside parentheses.  It should look like this:
 
           Enter the answer (default 42):
 
    • In ‘interactive’, if you use a Lisp expression to produce a list of
      arguments, don’t try to provide the correct default values for
      region or position arguments.  Instead, provide ‘nil’ for those
      arguments if they were not specified, and have the function body
      compute the default value when the argument is ‘nil’.  For
      instance, write this:
 
           (defun foo (pos)
             (interactive
              (list (if SPECIFIED SPECIFIED-POS)))
             (unless pos (setq pos DEFAULT-POS))
             ...)
 
      rather than this:
 
           (defun foo (pos)
             (interactive
              (list (if SPECIFIED SPECIFIED-POS
                        DEFAULT-POS)))
             ...)
 
      This is so that repetition of the command will recompute these
      defaults based on the current circumstances.
 
      You do not need to take such precautions when you use interactive
      specs ‘d’, ‘m’ and ‘r’, because they make special arrangements to
      recompute the argument values on repetition of the command.
 
    • Many commands that take a long time to execute display a message
      that says something like ‘Operating...’ when they start, and change
      it to ‘Operating...done’ when they finish.  Please keep the style
      of these messages uniform: _no_ space around the ellipsis, and _no_
      period after ‘done’.  Progress, for an easy way to generate
      such messages.
 
    • Try to avoid using recursive edits.  Instead, do what the Rmail ‘e’
      command does: use a new local keymap that contains a command
      defined to switch back to the old local keymap.  Or simply switch
      to another buffer and let the user switch back at will.  
      Recursive Editing.
 

Info Catalog elisp: Key Binding Conventions elisp: Tips elisp: Compilation Tips