Re: LMTX MkIV difference in expansion

[Wolfgang Schuster <wolfgang.schuster.lists@gmail.com>]

Rik Kabel schrieb am 20.11.2020 um 00:18:

> You are right about not quite understand.

There are cases where you want to pass a command to another command as 
it is without replacing it with its content, e.g. when you store the 
\TeX logo in the table of content the \TeX command should be written in 
the register and not the content of the command.

In the following example the first line prints the definition of the 
\TeX logo but in many cases you ant to preserve the command as in the 
second line.

\starttext

\tex{TeX} = \detokenize\expandafter{\TeX}

\blank

\tex{TeX} = \detokenize{\TeX}

\stoptext


To make it easier to keep the command eTeX added a new command 
\protected which can be used before \def to achieve this (ConTeXt 
provides the same thing under the name \unexpanded).

The following example shows how you can use \protected\def to keep 
always the current meaning of \foo when you print the content of \bar.

\starttext

\def\foo{foo}

\edef\bar{\foo}

\def\foo{bar}

\startlines
bar=\bar
foo=\foo
\stoplines

\blank

\protected\def\foo{foo}

\edef\bar{\foo}

\protected\def\foo{bar}

\startlines
bar=\bar
foo=\foo
\stoplines

\stoptext


A problem in older TeX engines is that \csname ...\endcsname didn't 
respect this protection and replaced the protected command with its 
content, recently Hans changed this behavior in LMTX which lead to the 
error message in your document.


> Does this mean that I can have the same definitions in MkIV and LMTX 
> (after some future update), or should I hunt down the \defines in both, 
> or that I should fork (or mode test) my source environment files, one 
> set for LMTX and one for MkIV?


When you use \define to store arguments which are passed as arguments to 
other command you have to change this to \defineexpandable but its best 
to do this in MkIV and LMTX because protected commands are the wrong 
thing in this case. Even though it would work in MkIV in some cases you 
run into problems when you pass argument to Lua.

Wolfgang
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