Spaces added in the argument of a command

Question

Consider the MWE example.

\documentclass{article}
\begin{document}
\def\toot#1{%
   \def\content{#1}%
   \meaning\content%
}
\toot{\textsc{1}}

s


\end{document}

Why the result of \meaning\toto is \toot{\textsc {1}} and not \toot{\textsc{1}} (that have an impact on Reledmac/Reledmac: duplicating index entries

Answer 1

TeX does not save character strings in its memory but tokens. When the parameter of \toot in the line of your example

\toot{\textsc{1}}

is scanned, the tokenizer creates the tokens "textsc", "{", "1" and "}". When TeX is asked to print the tokens (using \meaning in your example) then it must solve the question how tokens of control-sequence type are printed. If the printed token control sequence name consists of letters then TeX prints a backslash in front of and a space after such token. This is decision of TeX’s author. The reason: the tokens "relax", "a" are printed as \relax a which is better than \relaxa.

Edit If you want to elaborate with catcodes, then an example follows. The parameter of the \toot macro is read (and tokenized) when catcodes of backslash and space are 12 (normal). So you can detect duplicate spaces and there are no control sequences. As soon as the parameter is read, the standard catcodes are set back (at \egroup). Then a second "normal" variant of the parameter is prepared using \scantokens because we expect that macro programmer needs to manipulate with both variants: "normal" and "space-respected". Finally the macro \tootB is started. It gets two parameters, first is "normal" variant of parameter and second is "space-respected" variant (where no control sequences are).

\def\toot{\bgroup \catcode`\\=12 \catcode`\ =12 \tootA}

\long\def\tootA#1{\egroup
   \scantokens{\def\tmp{#1}}%
   \expandafter\tootB\expandafter{\tmp}{#1}%
}
\long\def\tootB#1#2{%
  \def\tmp{#1}%
  \message{normal: "\meaning\tmp"}
  \def\tmp{#2}%
  \message{space-respected: "\meaning\tmp"}
}

\toot{\textsc{1}}

\toot{\textsc{ a b   c \par a\par1 \textbf {a}a}}

\toot{\textsc{ a b c \hskip1em a\space 1 \textbf {a}a}}

\bye

Answer 2

With a macro which eats every space following a control sequence (so a macro name which consists of only alphabetic characters) which is not followed by an alphabetic character in its (once) expanded argument you can get what you want. It also eats the space if you typed it yourself, though.

EDIT: One could read the argument to the macro verbatim and then replace everything which should have been a macro with the macro. As a result \toottwo can't be used inside of an argument of another macro.

\documentclass[preview,border=2mm]{standalone}
\usepackage{xparse}
\ExplSyntaxOn
\tl_new:N \l_maieul_tl
\NewDocumentCommand \eatmacrospace { m }
  {
    \tl_set:No \l_maieul_tl { #1 }
    \regex_replace_all:nnN
      { (\\[A-Za-z]+)\s([^A-Za-z]) } { \1\2 } \l_maieul_tl
    \l_maieul_tl
  }
\str_new:N \l_maieul_str
\int_new:N \l_maieul_int
\NewDocumentCommand \toottwo { +v }
  {
    \tl_set:Nn \l_maieul_tl { #1 }
    \regex_replace_all:nnN { \\([A-Za-z]+) } { \c{\1} } \l_maieul_tl
    \regex_replace_all:nnN { \\([^A-Za-z]) } { \c{\1} } \l_maieul_tl
    \exp_args:NnV \regex_count:nnN { \{ } \l_maieul_tl \l_maieul_int
    \int_step_inline:nnnn { \c_one } { \c_one } { \l_maieul_int }
      {
        \regex_replace_all:nnN { \cO{(.*)\cO} } { \cB{\1\cE} } \l_maieul_tl
      }
    %\tl_show_analysis:N \l_maieul_tl % for debugging
    \let\content\l_maieul_tl
    \str_set:Nn \l_maieul_str { #1 }
    \texttt{macro:->\l_maieul_str}
  }
\ExplSyntaxOff
\long\def\toot#1{%
   \def\content{#1}%
   \texttt{\meaning\content}\\%
   \texttt{\eatmacrospace{\meaning\content}}%
}
\begin{document}
\toot{\textsc{1}}

\toot{\textsc{ a b c \par a\par1 \textbf {a}a}}

\toottwo{\textsc{ a b c \hskip1em a\space 1 \textbf {a}a}}

\content% content is expandable

s


\end{document}