Similar Structured Macros

Question

I am currently using the macro,

\def\<#1: #2: #3\>{\langle #1\;:\if#2\empty\else\;#2\;\fi:\;#3\rangle}

Taken from: http://ctan.org/pkg/tex-ewd.

And I would like to make another similar macro,

\def\<#1 if #2 else #3\>{ \langle #1\; \lhd #2\; \rhd \;#3\rangle }

But this latter one clashes with the former one. It seems I can only have one or the other. Any help on this matter would be greatly appreciated!

Thank-you!

PS. I do not know much about multiple deletmiters, or macros with such structure. Any help, or direction to learning more about this, would be most welcome!

Answer 1

You're looking for disgrace. A macro should do one job. However, here it is. The usage of \numexpr should be allowed with ConTeXt, but probably there's no need to set up fonts for using \lhd and \rhd

\def\<#1\>{\moseslookforif#1if\moseslookforif}
\def\moseslookforif#1if#2\moseslookforif{%
  \ifx\hfuzz#2\hfuzz
    % no if in the argument
    \mosescolon#1\mosescolon
  \else
    \mosesifelse#1if#2\mosesifelse
  \fi
}

\def\mosescolon#1: #2: #3\mosescolon{%
  \langle #1:\ifx\hfuzz#2\hfuzz\else#2\fi:#3\rangle
}
\def\mosesifelse#1 if #2 else #3if\mosesifelse{%
  \langle #1 \lhd #2 \rhd #3\rangle
}

%%% Code possibly to be omitted, if \lhd and \rhd are already available    
\font\tenlasy=lasy10
\font\sevenlasy=lasy7
\font\fivelasy=lasy5
\newfam\lasyfam
\textfont\lasyfam=\tenlasy
\scriptfont\lasyfam=\sevenlasy
\scriptscriptfont\lasyfam=\fivelasy
\mathchardef\lhd=\numexpr2*"1000+\lasyfam*"100+"01\relax
\mathchardef\rhd=\numexpr2*"1000+\lasyfam*"100+"03\relax
%%% end of code to possibly omit

% the example

$\<a : b : c\>$

$\<x if y else z\>$

\bye

Note that the test \if#2\empty is wrong. It will return true if the second argument to the macro is yy, for instance, which of course is not wanted.

I removed the excess spaces introduced with \; in the original macro.

Answer 2

You admit you are a beginner, so we'll start with an analysis of what you've proposed. When you say

\def\<#1: #2: #3\>{\langle #1\;:\if#2\empty\else\;#2\;\fi:\;#3\rangle}

you are defining a macro named \<, meaning that your .tex file must contain this token in order to invoke the macro. Further, the syntax you've defined says that your .tex input must invoke the macro with the following (very restrictive syntax): \<FIRSTARG:SPACE SECONDARG:SPACE THIRDARG\>. If you leave out the spaces, compilation will fail with an error. If any of the arguments themselves contain a space or colon, then that could likewise produce errors (or at list misplaced argument definitions).

So while you can define a macro this way, it seems not convenient for usage. More likely, you want the output to look a certain way, rather than requiring the typed input to appear a certain way. The normal way to define such a macro (in LaTex) for a 3-argument input would be \newcommand\macroname[3]{...macros to produce the desired output using inputs #1, #2, and #3...}. The invocation form is then \macroname{FIRSTARG}{SECONDARG}{THIRDARG}. In this way, braces delimit the inputs, which can now contain colons and spaces without ambiguity,

So if I defined \newcommand\colonangle[3]{\langle #1\;:\if#2\empty\else\;#2\;\fi:\;#3\rangle}, then I could obtain your desired first macro output with the syntax, for example, \colonangle{x}{y}{z}.

Now to your second example. Not only does it also create a macro named \< (which is a duplicate of the prior macro name, but now requiring a different input syntax), it requires you to use words like if and else in the input file. Instead, I propose a second macro definition, to avoid confusion, using a more standard input syntax.

\documentclass{article}
\usepackage{amssymb}
\newcommand\colonangle[3]{\langle #1\;:\if#2\empty\else\;#2\;\fi:\;#3\rangle}
\newcommand\arrowangle[3]{ \langle #1\; \lhd #2\; \rhd \;#3\rangle }
\begin{document}
$\colonangle{A}{B}{C} \ne \arrowangle{A}{B}{C}$
\end{document}