The expl3 syntax used by the developing programming layer for LaTeX3 is rather different from ‘traditional’ TeX syntax, and therefore needs to be turned on and off using the command pair \ExplSyntaxOn/\ExplSyntaxOff. In package code making use of expl3, the structure

\ExplSyntaxOn % Or implicit from \ProvidesExplPackage
....
\usepackage{somepackage}
....
\ExplSyntaxOff % Or the end of the package


will switch off expl3 syntax for the loading of somepackage and so will work whether this dependency uses expl3 or not.

This is achieved by using the LaTeX2e kernel mechanism \@pushfilename/@popfilename, which exists to deal with the status of @ but which is extended by expl3 to cover the new syntax too. However, this only applies as standard to code loaded using \usepackage (or the lower-level kernel command \@onefilewithoptions). Some bundles, most notable TikZ, provide their own loader commands for specialised files. These can be made ‘expl3-aware’ by including the necessary kernel commands

\def\myloader#1{%
\@pushfilename
\xdef\@currname{#1}%
% Main loader, including \input or similar
\@popfilename
}


For packages which also work with formats other than LaTeX, the push and pop steps can be set up using \csname

\def\myloader#1{%
\csname @pushfilename\endcsname
\expandafter\xdef\csname @currname\endcsname{#1}%
% Main loader, including \input or similar
\csname @popfilename\endcsname
}


Of course, that will only work with LaTeX (the stack is not present in plain TeX or ConTeXt), but as the entire package idea is essentially a LaTeX one that should be a small problem.

## Reworking and exposing siunitx internals

I’ve been talking for a while about working on a new major version of siunitx. I’ve got plans to add some new features which are difficult or impossible to deliver using the v2 set up, but here I want to look at perhaps what’s more important: the back end, programming set up and related matters.

I’ve now made a start on the new code, working first on what I always think of as the core of siunitx: the unit processor. If you take a look at the new material and compare it with the existing release the first thing that should be obvious is that I’ve finally made a start on splitting everything up into different sub-parts. There are at least a couple of reasons for this. First, the monolithic .dtx for v2 is simply too big to work with comfortably. More importantly, though, the package contains a lot of different ideas and some of them are quite useful beyond my own work. To ensure that these are available to other people, it would seem best to make the boundaries clear, and separate sources helps with that.

That leads onto the bigger picture change that I’m aiming for. As regular readers will know, I wrote the first version of siunitx somewhat by accident and in an ad hoc fashion. Working on v2, I decided to make things more organised and also to use expl3, which I’d not really looked at before. So the process of writing the second version was something of a learning experience. At the same time, expl3 itself has firmed up a lot over the time I’ve been working with it. As such, the current release of siunitx has rather a lot of rough edges. In the new code, I’m working from a much firmer foundation in terms of conventions, coding ideas and testing implementations. So for v3 I’m aiming to do several things. A key one for prospective expl3 programmers is the idea of defined interfaces. Rather than making everything internal, this time I’m documenting code-level access to the system. That means doing some work to have clearly defined paths for information to pass between sub-modules, but that’s overall a good thing. I’m also using the LaTeX3 teams new testing suite, l3build, to start setting up proper code tests: these are already proving handy.

The net result of the work should be a better package for end users but also extremely solid code that can be used by other people. I’m also hopeful that the ideas will be usable with little change in a ‘pure’ LaTeX3 context. Documenting how things work might even have a knock-on effect in emulating siunitx in say MathJax. Beyond that, I’ve viewed siunitx as something of a sales pitch for expl3, and providing a really top-class piece of code is an important part of that. If I can get the code level documentation and interfaces up to the standard of the user level ones, and improve the user experience at the same time, I think I’ll be doing my job there.

## Lua for LaTeX3 build scripts

Anyone who follows the LaTeX3 GitHub pages will have seen a lot of checkins recently, not only for the code itself but also for a new set of build scripts. The work should hopefully make us more efficient but also impact on others, directly or indirectly.

Once of the first things I did when I joined the LaTeX3 Project was to write a load of Windows batch scripts for building the code. At the time, most of the rest of the team were using Unix (Linux, Macs, …) but I was using Windows, so this was quite important. Since then, the team’s IT set up has varied a bit but so have the requirements for our scripts, so I’ve ended up working quite a bit with both the batch files and the Unix Makefiles.

The ‘two sets of scripts’ approach has a few problems. The most obvious is that we’ve had to keep the two in synch: not always entirely successful, and not always remembered! At the same time, the ‘standard’ facilities available on the two systems are different: we’ve had to require Perl on all platforms, and work out how best to make everything ‘looks the same’.

Some recent discussion prompted me to consider some new work on the scripts, but with Lua now available anywhere that has a TeX system (as texlua for script work), it seemed natural to move to an entirely new set up. The plan has been to have one ‘master’ script for the entire LaTeX3 repository, rather than the ‘copy-paste’ arrangement we’ve had to date, and to use very simple ‘configuration’ files for each separate module. That’s worked really well: we’ve now got one big file covering almost everything, no longer need Perl and have also addressed some of the things that prompted this in the first place! (More on those in another post soon.)

Even if you are not using LuaTeX for day-to-day TeX work, the scripting ability for supporting development is very handy. Lua doesn’t cover the OS-dependent stuff, but using our existing script code and a small amount of detection we’ve been able to work with that. The new scripts are hopefully nice and clear, more flexible than the old ones and most importantly only need changing in one place for each issue.

So how does this impact on others? First, it makes it easier for the team to work on LaTeX3, which should be a good thing all round. The scripts should also mean that we don’t have the odd strange change depending on which operating system is used to do a release. Second, we’d like to hope that other TeX developers can take ideas from our scripts for their own work. We’re particularly interested in testing TeX code, and I’ll come back to that in my next post. Last, and linked to testing, we are picking up some ‘interesting’ engine-based issues. Those are getting reported, and even if you never use LaTeX we’d hope that will be a benefit!

## Programming LaTeX3: A summary so far

Use of the LaTeX3 programming layer ‘expl3’ in LaTeX2e packages is continuing to grow. Ideally, the team would have a nice easy-to-follow Programming LaTeX3 book to support this, but as the language is still developing and because writing such a guide is hard we are not in that position just at the moment. At the moment, the nearest we get is a series of posts I wrote here a little while ago now, trying to at least put some basics down. What I’d like to build up is a ‘self-contained’ story, based on the idea that the reader knows how to use LaTeX as an ‘end user’ (making documents), but with no particular assumptions about programming. I’m hoping to look again at some of these topics in the coming months, but for the moment it seems like a good idea to summarise what we have so far.

To date, the series covers:

The posts run basically in order, so if you are looking to learn then start with the first one and work your way down.

Of course, looking back I think some revision would be handy: for example, now we have an FPU it might be useful to cover that around the same point as integers. I guess what might be sensible is to look seriously at putting together something more structured which can be revised: perhaps it is time for that book!

## Updates to expl3, xpackages and siunitx

A quick note to say that I’ve just uploaded to CTAN updates for expl3, xpackages and siunitx. There are some low-level changes which mean that these have to be updated together. So if you grab one of them, grab them all!