[texhax] A macro programming question

[Philipp Stephani]

Am 11.04.2010 um 15:19 schrieb Vafa Khalighi:

> 
> 
> 
> To add to the other solutions... this one only defines one
> macro and works with both { } and \bgroup \egroup...
> 
> \def\hboxR{\begingroup
>  \everyhbox{%
>   \aftergroup\endgroup
>   \everyhbox{}%
>   \beginR
>   \bgroup
>   \aftergroup\endR
>   \aftergroup\egroup
>  }\hbox}
> 
> 
> \setbox0=\hboxR{This is a test.}
> \box0
> 
> Would not work here, because TeX expects a box. How can one fix this?

The following is a hack that might or might not work in your situation:

\TeXXeTstate=1
\def\hboxR{\hhboxR{}}
\def\hhboxR#1#2{%
  \hhhboxR{#1}#2\nil
}
\def\firstofthree#1#2#3{#1}
\def\secondofthree#1#2#3{#2}
\def\thirdofthree#1#2#3{#3}
\def\hhhboxR#1#2#3\nil{
  \ifx&#3&%
    \ifx#2\bgroup
      \expandafter\expandafter\expandafter\firstofthree
    \else
      \expandafter\expandafter\expandafter\secondofthree
    \fi
  \else
    \expandafter\thirdofthree
  \fi
  {\hhhhboxR{#1}}%
  {\hhboxR{#1#2}}%
  {\hhhhhboxR{#1}{#2#3}}%
}
\def\hhhhboxR#1#2\egroup{%
  \hhhhhboxR{#1}{#2}%
}
\def\hhhhhboxR#1#2{%
  \hbox#1{\beginR#2\endR}%
}
\setbox0=\hboxR to 35pt \bgroup abc\hfil def\egroup
\box0
\bye

Issues: Significant spaces are gobbled, braces must be either both explicit or both implicit, only \bgroup...\egroup is allowed (no other implicit brace tokens), the contents are read as a macro argument, preventing interesting applications like opening and closing the box in different macros.
Maybe you want to abandon the idea of simulating the box primitives and use a different syntax (like LaTeX's \sbox and lrbox) instead.