outline-minor-mode is very useful when used in conjunction with cperl-mode, especially when editing large source files. It allows you to suppress the display of most of the file, allowing you to focus on the area you’re editing, and to navigate quickly to other areas.

However, in the default configuration, it’s very clunky and difficult to use. This page describes customizations that make outline-minor-mode and cperl-mode sing together.

Before you read further, you should go read up on OutlineMinorMode if you’re not already familiar with it, otherwise much of this page won’t make sense.

Redefining the keymap

The first thing to do is to change the keybinding for outline-minor-mode. By default it is ‘C-c @’, and all of the key sequences it uses are control keys. So you wind up typing ‘C-c @ C-n’ and ‘C-c @ C-p’ and so on. This ties your fingers up in knots, and all the vi users in the office will laugh at you, as it confirms all their stereotypes about emacs.

Here’s an excerpt from my .emacs file. I chose M-o as the prefix key for the outline functions, as it is not assigned to anything by default, and ‘o’ is a fine mnemonic for ‘outline’. Also, I replaced the control-key sequences with non-control alphabetic characters; much easier to type.

; Outline-minor-mode key map
(define-prefix-command 'cm-map nil "Outline-")
(define-key cm-map "q" 'hide-sublevels)    ; Hide everything but the top-level headings
(define-key cm-map "t" 'hide-body)         ; Hide everything but headings (all body lines)
(define-key cm-map "o" 'hide-other)        ; Hide other branches
(define-key cm-map "c" 'hide-entry)        ; Hide this entry's body
(define-key cm-map "l" 'hide-leaves)       ; Hide body lines in this entry and sub-entries
(define-key cm-map "d" 'hide-subtree)      ; Hide everything in this entry and sub-entries
(define-key cm-map "a" 'show-all)          ; Show (expand) everything
(define-key cm-map "e" 'show-entry)        ; Show this heading's body
(define-key cm-map "i" 'show-children)     ; Show this heading's immediate child sub-headings
(define-key cm-map "k" 'show-branches)     ; Show all sub-headings under this heading
(define-key cm-map "s" 'show-subtree)      ; Show (expand) everything in this heading & below
(define-key cm-map "u" 'outline-up-heading)                ; Up
(define-key cm-map "n" 'outline-next-visible-heading)      ; Next
(define-key cm-map "p" 'outline-previous-visible-heading)  ; Previous
(define-key cm-map "f" 'outline-forward-same-level)        ; Forward - same level
(define-key cm-map "b" 'outline-backward-same-level)       ; Backward - same level
(global-set-key "\M-o" cm-map)

Overriding cperl-mode's outline regexp

How does outline-minor-mode decide what is a heading and what is body text? How does it decide what the outline hierarchy is? It uses a regexp called outline-regexp and a function called outline-level.

The default outline regexp provided by cperl-mode is workable, but not great. It matches the following types of lines as heading lines:


It puts package at the top level, sub and =head1 as the second level, and =head2 as the lowest hierarchy level.

This seemed very odd to me. package should obviously be higher than sub, but =head1 should be higher than sub. And does anyone use =head2? It’s more useful to put a level in there for =item.

The above is simply my own personal thought process. Once I came up with my own hierarchy, I could customize cperl-mode’s outline-regexp and outline-level to work the way I wanted. You can come up with your own hierarchy and implement it your own way, too.

The hierarchy I settled on was:


The code follows (again, from my .emacs file):

(setq my-cperl-outline-regexp
       "^"                              ; Start of line
       "[ \\t]*"                        ; Skip leading whitespace
       "\\("                            ; begin capture group \1
       (join "\\|"
             "=head[12]"                  ; POD header
             "package"                    ; package
             "=item"                      ; POD item
             "sub"                        ; subroutine definition
       "\\)"                            ; end capture group \1
       "\\b"                            ; Word boundary

Aside: join, above, is a macro that works like Perl’s join:<br /> &nbsp;&nbsp;&nbsp;&nbsp;(defmacro join (join-char &rest others) `(mapconcat 'identity ',others ,join-char))

Now, I needed to override cperl-mode’s outline-level function – but if I do that in my .emacs file, then when cperl-mode gets loaded, it’ll clobber my own definition. So I had to delay the definition until after cperl-mode gets loaded, via cperl-mode-hook:

(setq cperl-mode-hook 'my-cperl-customizations)

(defun my-cperl-customizations ()
  "cperl-mode customizations that must be done after cperl-mode loads"

  (defun cperl-outline-level ()
    (looking-at outline-regexp)
    (let ((match (match-string 1)))
       ((eq match "=head1" ) 1)
       ((eq match "package") 2)
       ((eq match "=head2" ) 3)
       ((eq match "=item"  ) 4)
       ((eq match "sub"    ) 5)
       (t 7)

  (setq cperl-outline-regexp  my-cperl-outline-regexp)
  (setq outline-regexp        cperl-outline-regexp)
  (setq outline-level        'cperl-outline-level)

Note that you can do any sort of customizations you want in this function of yours – for example, I use some abbreviations in my Perl coding, so I have it automatically load abbrev-mode when cperl-mode starts.

Getting hairy

I thought it might be neat to outline some of the internals of subroutines as well, to show their control structures. So I redefined my regexp as follows:

(setq my-cperl-outline-hairy-regexp
       "^"                              ; Start of line
       "[ \\t]*"                        ; Skip leading whitespace
       "\\("                            ; begin capture group \1
       (join "\\|"
             "=head[12]"                  ;     POD header
             "package"                    ;     package
             "=item"                      ;     POD item
             "sub"                        ;     subroutine definition
             "if" "else" "unless" "while" "until" "return"
       "\\)"                            ; end capture group \1
       "\\b"                            ; Word boundary

Once I tried this, I didn’t like it, so I went back to the original. But I include it here to show how easy it is to customize the outline settings once you have the framework in place.

So what's outline-mode good for?

When editing large source files – or sometimes even small ones – the first thing I do is ‘M-o t’. This hides all of the text in the file except for the “header” lines, as I’ve defined them above. The source file then looks something like this:

=head1 NAME...

=head1 SYNOPSIS...


package Foo::Bar...

=head1 METHODS...

=item new...

sub new...

=item some_method...

sub some_method...

and so on. The ...s tell you that there is elided text there. To work on a given section of code, you move there (via mouse, the usual cursor keys, or the outline mode navigation key sequences), and type ‘M-o e’ to make the code visible. When done, leave it visible, or use ‘M-o c’ to hide it again.

Again, see the documentation for outline-minor-mode to learn all of the power of this mode. Suffice it to say that it makes navigating around in a large source file, or working with two separate areas of a large file, simple and easy.

Known Issues

1. PDE mode:

If you run PerlDevelopEnvironment, then you must wrap cperl-mode-hook into eval-after-load. Otherwise it will not take effect after pde-load.el is loaded.

(eval-after-load 'pde-load
  '(add-hook 'cperl-mode-hook (lambda ()
                                (outline-minor-mode 1))))