\noexpand, \unexpanded and \expandafter

Question

Recently I discovered a piece of code (from ucharclasses) using three expansion-related macros \noexpand, \unexpanded and \expandafter:

\def\do#1{\noexpand\setTransitionsFor{#1}{####1}{####2}}
\def\doclass#1{
  \begingroup\edef\x{\endgroup
    \noexpand\newcommand
    \unexpanded\expandafter{\csname setTransitionsFor#1\endcsname}[2]%
    {\csname #1Classes\endcsname}}\x}
\ClassGroups

I can find some references about these macros, but I still don't understand what they are doing exactly. I'd like to know why they are necessary in the above piece of code, and why would we ever need four sharps as in ####1 and ####2.

Here's an excellent tutorial on \expandafter: TUGBoat 9(1):57-61, 1988. — John, Jun 26 '20 at 20:38
@John OK. So \expandafter knows about \csname and \endcsname. That's a case explicitly listed. But in this piece of code there's unexpanded before \expandafter that still looks confusing. — Cyker, Jun 26 '20 at 20:44
David Carlisle has given a great answer, below. To reply to your comment, the role of \unexpanded is discussed in this thread. — John, Jun 26 '20 at 21:37

David Carlisle · Accepted Answer · 2020-06-26T21:23:31.737

2

\do shows \noexpand and ##

\def\do#1{\noexpand\setTransitionsFor{#1}{####1}{####2}}
\edef\z{\do{abc}}
\show\z

this shows

> \z=macro:
->\setTransitionsFor {abc}{##1}{##2}.
l.4 \show\z

So you see that in defining \z with \edef the csname \setTransitionsFor was prevented from being expanded by the \noexpand , #1 got replaced by the argument to \do which is abc here and ## got replaced by #

For the second macro, adding \show\x then running this fragment with pdflatex

\def\doclass#1{
  \begingroup\edef\x{\endgroup
    \noexpand\newcommand
    \unexpanded\expandafter{\csname setTransitionsFor#1\endcsname}[2]%
    {\csname #1Classes\endcsname}}%
    \show\x
    \x}
\doclass{abc}

produces

> \x=macro:
->\endgroup \newcommand \setTransitionsForabc [2]{\abcClasses }.
\doclass ...csname #1Classes\endcsname }}\show \x 
                                                  \x 
l.9 \doclass{abc}
? x

so after the \show the temporary macro \x will be executed and be equivalent to

\newcommand \setTransitionsForabc [2]{\abcClasses }

so the string abc that was passed to \doclass has been used to construct both the commandnames \setTransitionsForabc and \abcClasses

edited Jun 26 '20 at 21:23

answered Jun 26 '20 at 21:17

David Carlisle

757,742

1

Can we say \noexpand protects the next one token, while \unexpanded protects the entire token list between { and }? – Cyker Jun 27 '20 at 07:22
1

@Cyker yes, exactly.noexpand is classic Tex , unexpanded is an etex addition. – David Carlisle Jun 27 '20 at 07:25
And there seems to be another trick in this code: The use of \begingroup and \endgroup is meant to hide the definition of \x so that it doesn't pollute the current scope? – Cyker Jun 27 '20 at 07:26
@Cyker you are getting the hang of this:) yes – David Carlisle Jun 27 '20 at 07:38

\noexpand, \unexpanded and \expandafter

1 Answers1