Elastic Modes

This is an Emacs package for text/code alignment for variable-width (aka proportional, or variable pitch) fonts.

Purpose

Two minor modes for Emacs, elastic-indent and elastic-table, automatically alter the width of certain spaces and/or tabs in the buffer to align buffer text. The two modes operate independently and can be used separately or together. They serve two purposes, broadly speaking:

1. With a variable-width font, these modes provide some of the alignment functionality that you would otherwise have gotten for free by counting characters in a fixed-width (monospace) font. This is the main motivation for them.
   • Using elastic-indent, the initial spaces on a line will have their width adjusted to match the same number of characters on previous lines, so that the left edge of the text/code will be aligned as it would be in a fixed-width font.
   • Using elastic-table, you can use tabs to align text in other places than the left edge. elastic-table is essentially an implementation of elastic tabstops.
2. Certain features may be useful even with fixed-width fonts:
   • Using elastic-table you can quickly create automatically-aligned tables in text or code using tab characters as the column separator.
   • Using elastic-indent with spaceship-auto-preserve (considered experimental and disabled by default), aligned code blocks will have their alignment preserved in certain situations when the code they are aligned to changes position during editing.

Caveats

Non-standard space/tab conventions.

You must use spaces and tabs consistently in a way that supports the mode. In particular, with elastic-indent you must strictly adhere to the convention of using tabs for indentation and spaces for alignment; otherwise it won’t work.

No user interface

This code is provided as a backend only, and relies on conventions that are contradictory to the assumptions of stock Emacs. It will take some effort to integrate it into your workflow. See example setup below for some clues on how to get started.

No promises

At time of release this code is untested with most major modes and popular Emacs extensions. Issue reports are welcome, but please read the next section carefully first to be sure you understand the expected behavior. Not every programming languages as a syntax compatible with these conventions.

What It Does, Precisely

elastic-indent

Certain spaces have their widths adjusted to match that of another character on the previous line; generally speaking the nth initial space will match the nth character on the previous line, which might itself be an initial space that matches something further back. To be precise, a character matches another character exactly when:

• All previous characters on the same line match something, and
• There is a corresponding character on the previous line and it is eligible to be matched by this character:
  • A tab can match only another tab.
  • A space can match any character except a tab.
  • Other characters do not match anything.

Each matching space has its width modified to be the same as the character it matches. This applies recursively: if the matched character’s width is itself modified, the modified width is matched. If a tab character is found in the indentation, then it is treated as the equivalent number of spaces that it normally stands for (depending on the `tab-width` variable).

The rules produce good results when only spaces are used for indentation, but they also work if tabs are used. The particular kind of mix found in the Emacs sources will also work, as well as the convention “tabs for indentation, spaces for alignment”.

elastic-table

Tabs which are not part of the leading space act as column separators. A maximal sequence of consecutive lines containing column separators and with the same leading-space form an elastic table. The elastic-table mode adjusts the width of all column separators so that the columns of each tabble are left-aligned. Note that the first cell of each row cannot be empty since otherwise there would be no column separators in that line and the table would be ended.

Note that, in a default Emacs configuration, typing TAB will not insert a tabulation character. You need to type C-q C-i to insert a column separator.

Example Setup

A minimal setup with use-package may looks like so:

;; use a variable-pitch face in programming modes.
(add-hook 'prog-mode-hook
          (lambda ()
            (face-remap-add-relative 'default 'variable-pitch)))
;; (Customize variable-pitch face properly)

(use-package elastic-indent
  :hook (prog-mode . elastic-indent-mode))

(use-package elastic-table
  :hook (prog-mode . elastic-table-mode))

Known Issues

• Changing the font, or anything else that affects display widths without runnning after-change-functions or text-scale-mode-hook, can mess up the alignment. You can manually run the command elastic-tools-do-buffer to fix this.

Acknowledgements

This is a fork of Scott Messick’s spaceship and tabble modes. This package provides a more traditional treatment of indentation, and has more focus on efficient runtime behavior.