24.6 Outline Minor Mode

Outline minor mode is a buffer-local minor mode that hides parts of the buffer and leaves only heading lines visible. This minor mode can be used in conjunction with other major modes (see Outline Minor Mode in the Emacs Manual).

There are two ways to define which lines are headings: with the variable outline-regexp or outline-search-function.

Variable: outline-regexp ¶

This variable is a regular expression. Any line whose beginning has a match for this regexp is considered a heading line. Matches that start within a line (not at the left margin) do not count.

Variable: outline-search-function ¶

Alternatively, when it’s impossible to create a regexp that matches heading lines, you can define a function that helps Outline minor mode to find heading lines.

The variable outline-search-function specifies the function with four arguments: bound, move, backward, and looking-at. The function completes two tasks: to match the current heading line, and to find the next or the previous heading line. If the argument looking-at is non-nil, it should return non-nil when point is at the beginning of the outline header line. If the argument looking-at is nil, the first three arguments are used. The argument bound is a buffer position that bounds the search. The match found must not end after that position. A value of nil means search to the end of the accessible portion of the buffer. If the argument move is non-nil, the failed search should move to the limit of search and return nil. If the argument backward is non-nil, this function should search for the previous heading backward.

Variable: outline-level ¶

This variable is a function that takes no arguments and should return the level of the current heading. It’s required in both cases: whether you define outline-regexp or outline-search-function.

If built with tree-sitter, Emacs can automatically use Outline minor mode if the major mode sets the following variable.

Variable: treesit-outline-predicate ¶

This variable instructs Emacs how to find lines with outline headings. It should be a predicate that matches the node on the heading line.