How to switch macro expansion by giving optional arguments?

Question

Popular ways to define a macro with optional arguments is using TeX's \def with the aid of \@ifnextchar[ and/or LaTeX's \newcommand. In both cases, macro's definition usually contains its optional arguments.

Now, I want to use optional arguments just as a sort of junction which decides how macro expands. The option itself is not used in macro's definition explicitly.

For a simple instance, let me define \conjTrans, which denotes a conjugate transpose of a matrix in three notations. The specification of \conjTrans is as follows:

\documentclass{standalone}
\usepackage{conjTrans} % \conjTrans is defined here
\begin{document}
$\conjTrans{A}$,
$\conjTrans[asterisk]{B}$,
$\conjTrans[Hermite]{C}$, and
$\conjTrans[dagger]{D}$
\end{document}

will produce

,

and if invalid option (e.g. dager) is given, an error like the option is incorrect occurs.

I first tried to utilize \ifx or other conditional branches, but it didn't go well with my competence because options are more than one token and I don't know the solution to it.

I implemented \conjTrans with the following code.

% conjTrans.sty
\ProvidesPackage{conjTrans}[2016/01/05 my first question on TeX.SX]
\RequirePackage{amsmath}
\def\conjTrans{\@ifnextchar[\nkt@conjTrans{\nkt@conjTrans[asterisk]}}
\def\nkt@conjTrans[#1]#2{%
    \expandafter\ifx\csname nkt@conjTrans@#1\endcsname\relax
        \PackageError{conjTrans}{The option #1 is unknown to \string\conjTrans}{Optional argument for \string\conjTrans\space must be either ast, H, or dagger.}%
    \else
        \csname nkt@conjTrans@#1\endcsname#2%
    \fi
}
\def\nkt@conjTrans@asterisk#1{\boldsymbol#1^{\ast}}
\def\nkt@conjTrans@Hermite#1{\boldsymbol#1^{\text{H}}}
\def\nkt@conjTrans@dagger#1{\boldsymbol#1^{\dagger}}

This \conjTrans seems to work quite well, but I wonder if there is a method which is more sophisticated, safer, or better in certain sense than mine. Are there any standard “idioms” to use optional arguments in this way?

Answer 1

In this case I think a key-value approach is the best one:

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\conjTrans}{O{asterisk}m}
 {
  \yudai_conj_trans:nn { #1 } { #2 }
 }

\keys_define:nn { yudai/conjtrans }
 {
  asterisk .code:n = \tl_set:Nn \l_yudai_conj_trans_symbol { * },
  Hermite  .code:n = \tl_set:Nn \l_yudai_conj_trans_symbol { \mathrm{H} },
  dagger   .code:n = \tl_set:Nn \l_yudai_conj_trans_symbol { \dagger },
  asterisk .value_forbidden:n = true,
  Hermite  .value_forbidden:n = true,
  dagger   .value_forbidden:n = true,

  unknown  .code:n = \msg_error:nnx { yudai/conjtrans } { bad-option } { \l_keys_key_tl }
                     \tl_set:Nn \l_yudai_conj_trans_symbol { ??? },
 }

\msg_new:nnnn { yudai/conjtrans } { bad-option }
 {
  Bad~option~#1
 }
 {
  You~used~#1,~which~is~not~among~the~predefined~options
 }

\tl_new:N \l_yudai_conj_trans_symbol

\cs_new_protected:Nn \yudai_conj_trans:nn
 {
  \keys_set:nn { yudai/conjtrans } { #1 }
  #2^{\l_yudai_conj_trans_symbol}
 }
\ExplSyntaxOff

\begin{document}

$\conjTrans{A}$,
$\conjTrans[asterisk]{B}$,
$\conjTrans[Hermite]{C}$,
and $\conjTrans[dagger]{D}$

Error: $\conjTrans[foo]{E}$

\end{document}

This is easily extendable.

A “classical” implementation:

\documentclass{article}

\newcommand{\conjTrans}[2][asterisk]{#2\conjTransAppend{#1}}

\newcommand{\conjTransAppend}[1]{%
  \ifcsname conjTrans#1\endcsname
    ^{\csname conjTrans#1\endcsname}
  \else
    \PackageError{conjTrans}{Invalid option #1}
      {The option #1 is not among the predefined ones}%
    ^{???}%
  \fi
}
\newcommand{\conjTransasterisk}{*}
\newcommand{\conjTransHermite}{\mathrm{H}}
\newcommand{\conjTransdagger}{\dagger}

\begin{document}

$\conjTrans{A}$,
$\conjTrans[asterisk]{B}$,
$\conjTrans[Hermite]{C}$,
and $\conjTrans[dagger]{D}$

Error: $\conjTrans[foo]{E}$

\end{document}

Answer 2

\documentclass[a4paper]{article}
\usepackage{filecontents}
\begin{filecontents*}{conjTrans.sty}
\ProvidesPackage{conjTrans}[2016/01/05 my first question on TeX.SX]
\RequirePackage{amsmath}
\RequirePackage{xkeyval}
\define@choicekey*+{conjTrans}{symbol}[\val\nr]{,asterisk,Hermite,dagger}%
  {\ensuremath{\ifcase\nr\relax
    \cTB@se^*\or\cTB@se^\text{H}\or\cTB@se^{\dagger}\fi}}
  {\PackageWarning{conjTrans}{erroneous input ignored}}
\newcommand\conjTrans[2][asterisk]{\def\cTB@se{\boldsymbol{#2}}\setkeys{conjTrans}{symbol=#1}}
\endinput
\end{filecontents*}

\usepackage{conjTrans}
\begin{document}
\conjTrans{A}, \conjTrans[asterisk]{B}, \conjTrans[Hermite]{C}, and 
\conjTrans[dagger]{D} 

\conjTrans[whatever]{A}%% For an error message in the log file
\end{document}