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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.

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3
  • Here's an excellent tutorial on \expandafter: TUGBoat 9(1):57-61, 1988.
    – John
    Jun 26, 2020 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, 2020 at 20:44
  • 1
    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, 2020 at 21:37

1 Answer 1

Reset to default
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\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

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4
  • Can we say \noexpand protects the next one token, while \unexpanded protects the entire token list between { and }?
    – Cyker
    Jun 27, 2020 at 7:22
  • 1
    @Cyker yes, exactly.noexpand is classic Tex , unexpanded is an etex addition.
    – David Carlisle
    Jun 27, 2020 at 7:25
  • 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, 2020 at 7:26
  • @Cyker you are getting the hang of this:) yes
    – David Carlisle
    Jun 27, 2020 at 7:38

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