[Tex/LaTex] Space after command with \write

spacingwrite file

I did not see this question yet, correct me if there is.

It's not the usual problem of adding a space after a command. It's more specifically when writing to a file. For some reason the same method does not apply when using the \write command.

Minimal working example:




% write to pdf
\Insert stuff.\par % wrong method
\Insert\ stuff.\par   %
\Insert{} stuff.\par  % all valid methods
\Insert~stuff.\par    %

% open file to write to

% write the same to file
\writefile{\Insert stuff.}
\writefile{\Insert\ stuff.}
\writefile{\Insert{} stuff.}

% close file


Output pdf:

Insert stuff.
Insert stuff.
Insert stuff.

Output filename.txt

Insert\ stuff.
Insert{} stuff.
Insert\protect \unhbox \voidb@x \penalty \@M \ {}stuff.

Which is not quite what I need or expected.

Best Answer

TeX ignores spaces after control sequences (formed by letters) and performs complete expansion when doing a \write.

The tokens it sees when you do \writefile{\Insert stuff.} are (separated by • just for clarity)


and there's no space. The expansion gives


When you put a pair of braces, the space after them is not ignored (it doesn't follow a control sequence):

\Insert•{•}• •s•t•u•f•f•.

With \Insert\ stuff it's the same: the token "control space" is unexpandable, so you get

Insert\ stuff.

With \Insert~stuff you lose: the complete expansion of ~ is not what you'd expect: it's the set of instructions necessary to "print a non breaking space". Indeed LaTeX has \protected@write to cope with this kind of commands that should not be expanded during a write.

As zeroth explained, the expansion of \space is a space; but it's not ignored after \Insert, because when TeX is reading tokens it doesn't yet see the expansion of \space, but that token (which undoubtedly isn't a space token).

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