Emacs as an amazing LaTeX editor

The purpose of this post is to point you towards some great features and packages if you’re already using Emacs to edit LaTeX, and to make you jealous if you’re using some other editor¹.

This isn’t a tutorial for Emacs or even a tutorial on how to write LaTeX inside Emacs. Rather, it’s supposed to give an idea of what’s possible, either as inspiration or to convince you to give Emacs a try. One problem is that setting all of this up can be a huge time sink, so you might want to use a framework such as Doom, where you just need to enable the LaTeX module and get almost everything I describe here.

The basics

Of course you get all the basics you would expect from a LaTeX editor. Synctex is supported (meaning you can jump from a certain line in the LaTeX code to the corresponding place inside your PDF viewer and the other way around), you can compile files from inside emacs, you can jump to compilation errors if there are any, there is auto-completion and so on.

Visuals

LaTeX can produce beautiful documents, but the source code isn’t very readable when writing mathematical expressions:

  \alpha \mapsto \int_{\R}e^{-\alpha x^2}\,dx

Emacs and AUCTeX (which is the de-facto standard package for using LaTeX inside Emacs) have several features that improve this situation:

• preview-latex replaces equations (and other parts of the LaTeX document) with images by compiling them. This means they look exactly the same inside the editor as they will in the compiled document. When the cursor is on an equation, this image preview is automatically replaced by the underlying text so you can still easily edit equations. However, this method of course has a noticeable delay because it requires a call to the LaTeX compiler.
• LaTeX superscripts and subscripts are displayed as super-/subscripts inside the editor. This is a purely visual feature, editing them doesn’t require “entering” or “exiting” the subscript or anything like that.
• prettify-symbols-mode allows you to replace any string with any unicode symbol. AUCTeX comes with a fairly comprehensive predefined list, which replaces LaTeX commands such as greek letters, arrows and others with symbolic representations. But you can also add your own. For example, the example above uses \R, which my custom style file defines as \mathbb{R}, and it’s possible to add replacement rules for such custom commands (as long as there is a fitting Unicode symbol). This makes the line above look like this in my editor:

  

  When the cursor moves over one of those Unicode symbols, it is expanded to the underlying text. And the nice thing about this is that it’s essentially instantaneous because nothing needs to be compiled.
• Folding is something similar but more general (though unlike prettify-symbols-mode it’s specific to LaTeX). It doesn’t just allow replacing fixed strings but also more complicated expressions. By default, this is used for example to display \label{some_label} as [l] (which as always expands when the cursor moves over it). The reasoning here is that some elements such as labels are just distractions when reading LaTeX source code. But you can also use this to further improve how math is displayed, see this config for some ideas (and in general for more ideas on how to beautify LaTeX inside Emacs).

Editing

AUCTeX has a couple of nice features that make typing LaTeX a bit easier. For example, you can let it automatically insert braces {} when typing _ or ^ inside a math environment, you can let it insert \(\) when typing a dollar sign, and even \enquote{} when typing " ².

But things get even better with the evil-tex package. As the name suggests, this is only relevant if you’re using evil-mode (vim keybindings inside emacs), but if so, it’s definitely worth trying. Just a few examples of what this allows you to do:

• Say you’ve typed

      \(ax^{2} + b\)
  

  and suddenly realize that this is supposed to go into an exponent. With your cursor anywhere on this math environment, type ysim^ (“surround everything inside the math environment as an exponent”) and you’ll get

      \(^{ax^{2} + b}\)
  

  with the cursor at the ^. Now you just need to enter the base.

• Your equations is now

      \(e^{ax^{2} + b}\)
  

  and you decide that this merits its own displayed rather than inline equation. So you type csmee (“change the surrounding math environment to equation”) and get

      \begin{equation}
      e^{ax^{2} + b}
      \end{equation}
  

• After a bit more editing, you have (for some reason)

      \begin{equation}
      \beta(e^{ax^{2} + b} + \frac{1}{x})
      \end{equation}
  

  Of course this looks ugly in the compiled document, you need to use \left( and \right). With evil-tex, you can just type mtd (“toggle delimiter”) with the cursor anywhere inside the parantheses, and it will add \left and \right for you. Type mtd again to go back to just the parantheses.

Emacs calc’s embedded mode

calc is the built-in calculator for Emacs; though saying “calculator” is a bit misleading because it can do symbolic differentiation, unit conversion, linear algebra and more. If your press C-x * e with your cursor on any LaTeX equation, you will start calc in “embedded mode”. This means that calc will parse the LaTeX code and then let you do any calculations you want involving the expression. The result will automatically be converted back to LaTeX and written into the buffer.

For example, say you have

\[\sin\left( x^2 + \sqrt{x} \right)\]

and want to know the derivative. You can enter embedded mode and type ad to differentiate, then type x when prompted for the variable with respect to which to differentiate. And just like that, you will have

\[\left( 2 x + \frac{0.5}{\sqrt{x}} \right) \cos\left( x^2 + \sqrt{x} \right)\]

inside your buffer. calc can even parse and output things like \begin{pmatrix}...\end{pmatrix}, so you can multiply matrices as well.

And more

I’ve only covered some of my personal favorite features when it comes to writing LaTeX inside Emacs, there’s much more. For example, LaTeX-math-mode allows you to very quickly enter mathematical symbols and RefTeX as well as other packages make handling references, labels and citations very efficient. And of course there are a gazillion other packages that can make writing LaTeX easier – this is Emacs after all.

The downside is of course that there is a pretty steep learning curve. But for people who need to write LaTeX documents all the time, I’d argue it’s worth it.