I've read Differences between LuaTeX, ConTeXt and XeTeX, but still have a niggle at the back of my mind that I'd like to ask about.
I'm developing a LaTeX package/class/stuff with the aim of making it possible for me to write any document I like in LaTeX even if the output format is not PDF/DVI/PS (see Getting LaTeX on to the Web for the backstory). Because this is subverting TeX ever-so-slightly, I end up having to do a bit of tinkering under the bonnet. Because my TeX skills are somewhat basic, I often wonder if I'm going about things the right way. Let me give two pertinent examples.
There are several times where it is convenient to use catcode changes to implement some stuff. For example, making
^active so that I can preprocess the stuff that is about to get superscripted. But messing around with catcodes is part of the Dark Arts and it seems that wherever I actually do the catcode change I end up in trouble. One of the simplest bits of trouble is when a macro argument is read in but needs a catcode change in the middle. When I get to this point, I find myself thinking:
I could do this by cunning auxiliary macros and other
\aftergroupnonsense or ... I could say "Stuff it, I'll use
The eventual output is text, but it has to go through PDF and then a pdf-to-text converter. I want some ligatures, such as
--to an endash and quotes, but not most ligatures. As the eventual output is text the actual font doesn't matter so I could create my own that suits. When I get to this point, I find myself thinking:
I could spend ages sorting out all the
.ttfnfiles or ... I could say "Stuff it, I'll use
The point is that if I insist on using a particular engine, I could go for the easy options.
Now, the primary purpose of my package is to make it easy for me to write documents using my familiarity with LaTeX. It is not not not for converting existing LaTeX documents to some other format. Nonetheless, I would like to make it useful for other people to use, and experience says that the first use most will put it to is to convert some existing document to some other format (who reads the manual these days?).
So assuming that I put in some instructions on which packages to add or remove (thinking mainly of fonts), my question is this: is there anything in terms of the basic programming functionality of the different engines that I need to be aware of?
In more direct terms, if I think purely with regard to primitives, is there a total order on the engines: TeX < eTeX < XeTeX < LuaTeX? Or, is there anything that TeX can do that, say, LuaTeX can't?