LaTeX2(e) vs LaTeX3

Question

I have some questions regarding the different versions of LaTeX.

1. Is LaTeX3 fully backwards compatible?
2. Is LaTeX2(e) deprecated?
3. Which version of Latex do I get when I install MiKTeX or TeX Live? 2 or 3?
4. What new features are added in LaTeX3?

Answer 1

There is no LaTeX3. At least in October 2022 when this answer is being written. And there will be no LaTeX3 in the next few years, if ever.

For several years, LaTeX kept on version 2.09 (the last one authored and maintained by Leslie Lamport). However developments were necessary in order to improve font access and to allow integration of AMS-TeX in LaTeX. So Lamport handed maintenance to a team led by Frank Mittelbach and in some time a definitive version of LaTeX was issued, called LaTeX2_ε. The ε was meant to denote that such version was final apart from bug fixes (it's a mathematical joke, because ε is used to denote the first uncountable ordinal, so something which is, in some sense, unreachable). This was in 1995.

Work on a future version of LaTeX, called LaTeX3, started soon thereafter, but was stalled for a long time because of hardware limitations: still in the first years of the third millennium, personal computers were too slow in order that an implementation of the (very neat) ideas of the team could reasonably work.

As years passed, hardware limitations faded away and, also due to recruiting new team members (notably Joseph Wright and Bruno Le Floch), development restarted.

The idea was that LaTeX3 should be based on a “new programming layer” that was made more efficient and powerful. On the top of this layer, the kernel and packages could be written more easily and avoiding clashes, by providing tools for the most common needed structures. (When reading package code, one is often surprised of how many times the same programming tool is recreated, with different names; and sometimes in ways that harm other packages.)

In the meantime, LaTeX2_ε got developed, because of new needs (think to Unicode support and to the new engines pTeX, XeTeX and LuaTeX, for instance). And more and more packages were contributed on CTAN.

A “pure” LaTeX3 kernel would make loading LaTeX2_ε packages without changes nearly impossible unless such kernel has all of the LaTeX2_ε kernel incorporated.

So, a few years ago, the team decided to abandon the idea of a brand new LaTeX3 and to gradually integrate the new programming layer (now called expl3) in the LaTeX2_ε kernel.

Nowadays, more and more packages exploit the expl3 programming layer and the trend will increase in the future.

To answer your questions:

1. there is no LaTeX3;
2. LaTeX2_ε is state-of-the-art;
3. you get the only version available, that is, LaTeX2_ε;
4. every six months, new features and bug fixes are being added to LaTeX2_ε; you can consult ltnews (texdoc ltnews) to find them and to get pointers to renewed documentation. The team is always very careful not to introduce backwards incompatibilities.