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How to break the 9-argument limit

If you think about it, you will realise that Knuth’s command definition syntax:

\def\blah#1#2 ... #9{<macro body>}

is intrinsically limited to just 9 arguments. There’s no direct way round this: how would you express a 10th argument? — and ensure that the syntax didn’t gobble some other valid usage?

If you really must have more than 9 arguments, the way to go is:

\def\blah#1#2 ... #9{%
  \def\ArgI{{#1}}%
  \def\ArgII{{#2}}%
  ...
  \def\ArgIX{{#9}}%
  \BlahRelay
}
\def\BlahRelay#1#2#3{%
  % arguments 1-9 are now in
  %   \ArgI-\ArgIX
  % arguments 10-12 are in
  %   #1-#3
  <macro body>%
}

This technique is easily extendible by concert pianists of the TeX keyboard, but is really hard to recommend.

LaTeX users have the small convenience of merely giving a number of arguments in the \newcommand that defines each part of the relaying mechanism: Knuth’s restriction applies to \newcommand just as it does to \def. However, LaTeX users also have the way out of such barbarous command syntax: the keyval package. With keyval, and a bit of programming, one can write really quite sophisticated commands, whose invocation might look like:

\flowerinstance{species=Primula veris,
  family=Primulaceae,
  location=Coldham's Common,
  locationtype=Common grazing land,
  date=1995/04/24,
  numplants=50,
  soiltype=alkaline
}

The merit of such verbosity is that it is self-explanatory: the typist doesn’t have to remember that argument twelve is soiltype, and so on: the commands may be copied from field notes quickly and accurately.

keyval.sty
Distributed as part of macros/latex/required/graphics (or browse the directory); catalogue entry

* faq-mac-prog.tex (q-keyval): tweak words about getoptk


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URL for this question: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=moren9

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This is FAQ version 3.28, released on 2014-06-10.