Snap-indent

Synopsis

Snap-indent provides simple automatic indentation (and optional formatting) when yanking/pasting text. It was inspired by auto-indent-mode, and is designed for improved simplicity, flexibility, and interoperability.

snap-indent-mode is an Emacs minor mode that enables the following features:

• Indent inserted text according to major mode on yank/paste
• Indent buffer text according to major mode on save (optional)
• When indenting, additionally format text, e.g. tabify, untabify, remove trailing whitespace, etc (optional)
• Prevent minor mode activation in certain major modes (optional)
• Skip indentation systematically by maximum text length or according to any user-defined predicate (optional)
• Skip indentation for a single operation using an argument prefix (optional)

Snap-indent's additional formatting behavior is very flexible. Any function that operates on a region may be used, and multiple functions may be specified.

Snap-indent can be configured to skip indentation with equal flexibility. Any predicate function can be set control this behavior systematically, and indentation may be suppressed for a single operation with key input.

Related packages

Complement to electric-indent-mode

electric-indent-mode indents while typing. Snap-indent indents on yank/paste and optionally on save. These do not overlap, and work well together.

Successor to auto-indent-mode

Snap-indent was originally created as a minimalist replacement for auto-indent-mode. That package was widely used and appreciated, but also quite complex, which led to interoperability bugs with other common packages. In contrast, snap-indent, while quite flexible, is designed for simplicity and hygiene. It's lightweight and should play well with other packages.

As of 2023-06-23, auto-indent-mode was retired by its author, who recommended snap-indent as a successor.

Installation

Snap-indent is available from MELPA.

Usage

Execute M-x snap-indent-mode to enable in any buffer, or add a hook to enable for a specific mode:

(add-hook 'prog-mode-hook #'snap-indent-mode)

To configure via use-package, adapt the following example as desired:

(use-package snap-indent
  :hook (prog-mode . snap-indent-mode)
  :custom ((snap-indent-format 'untabify)
           (snap-indent-on-save t)))

Customization

The following customization variables are available:

Variable	Type	Default	Description
snap-indent-excluded-modes	symbol list	See list	Major modes in which to ignore activation
snap-indent-format	function or list	nil	Additional formatting to apply when indenting
snap-indent-on-save	boolean	nil	Whether to indent the entire buffer on save
snap-indent-length-limit	integer	nil	Maximum text length to indent
snap-indent-skip-on-prefix-arg	boolean	nil	Whether a prefix argument causes indentation to be skipped
snap-indent-skip-on-condition	function	nil	Predicate function to cause indentation to be skipped

Additional formatting

Snap-indent can optionally apply additional formatting when indenting. This is highly customizable and controlled by the variable snap-indent-format. When non-nil, this variable may be either a single function or a list of functions to apply sequentially:

(setq snap-indent-format 'untabify)                                  ; single function
(setq snap-indent-format '(untabify delete-trailing-whitespace ...)) ; list of functions

Each function must accept two arguments: the beginning and end positions of the region on which to operate. Functions may be specified as symbols or lambda forms. Useful built-in functions include tabify and untabify for tab/space conversion and delete-trailing-whitespace.

Controlling activation

• Prevention by major mode. To prevent enabling of snap-indent-mode in certain major modes, include these in the snap-indent-excluded-modes list. This permits activation for prog-mode while excluding certain modes derived from prog-mode, for example.

• Skipping individual operations. When snap-indent-mode is enabled, its indentation behavior may still be skipped for an individual operation.

  • To skip indentation on large blocks for text (for performance reasons), set snap-indent-length-limit.
  • To skip indentation manually for a single command, set snap-indent-skip-on-prefix-arg to t, then use a prefix argument (e.g. C-u) when invoking the command.
  • To skip indentation systematically according to any user-specified logic, set snap-indent-skip-on-condition to a predicate function. The function must accept two arguments, which specify the start and end positions of the region on which to (potentially) operate. The function should return non-nil to skip indentation, and nil otherwise.

License

Copyright © 2023 Jeff Valk

Distributed under the GNU General Public License, version 3