Btukfyl

I am writing styled text to an external file (using the newfile package). I can successfully style the text with many attributes, e.g., largemdseriesitshape. But, very puzzling to me, when I style the text with footnotesize, I get an unexpected "Undefined control sequence" error, and I can't tell why it is arising:

(/compile/output.nameofstream

! Undefined control sequence.

l.1 ... {10}{12}selectfont abovedisplayskip 10p

                                                  @ plus2p@ minus5p@ abov...

The following MWE compiles fine and generates appropriately styled text both in the external file and in the output. The external file contains:

relax fontsize {14.4}{18}selectfont mdseries itshape Text to be styled

But when I uncomment out renewcommand{styleForText}{footnotesize}, I get the above error message and the external file contains:

relax fontsize {10}{12}selectfont abovedisplayskip 10p@ plus2p@ minus5p@ abovedisplayshortskip z@ plus3p@ belowdisplayshortskip 6p@ plus3p@ minus3p@ def leftmargin leftmargini parsep 5p@ plus2.5p@ minusp@ topsep 10p@ plus4p@ minus6p@ itemsep 5p@ plus2.5p@ minusp@ {leftmargin leftmargini topsep 6p@ plus2p@ minus2p@ parsep 3p@ plus2p@ minusp@ itemsep parsep }belowdisplayskip abovedisplayskip Text to be styled

I never expected that footnotesize would result in such different behavior than large. What's going on?

documentclass[12pt]{article}

usepackage{newfile}

newcommand{streamName}{nameofstream}

newcommand{sendStyledTextToStream}[1]{%

    addtostream{streamName}{styleForText#1}%

}

newcommand{printStream}{%

    closeoutputstream{streamName}%

    input{jobname.streamName}%

}

newcommand{styleForText}{largemdseriesitshape}

%renewcommand{styleForText}{footnotesize}

begin{document}

newoutputstream{streamName}

openoutputfile{jobname.streamName}{streamName}

sendStyledTextToStream{Text to be styled}

printStream

end{document}

I'm answering my own question in order to bring closure. A commenter (@GuM) answered it in a comment, but never converted it into an answer.

His solution: Use protectfootnotesize.

He gave the further explanation:

It’s best to protect the large declaration too; it doesn’t exhibit
the same problem, in this particular situation, just because its
expansion doesn’t happen to contain “internal” commands using the @
character, contrary to footnotesize. On the other hand, you might
have uncovered a bug: several years ago, the commands for changing the
font style (e.g., itshape, mdseries…) were made robust so that they
no longer required to be protected; but apparently, they forgot to
apply a similar change to the commands that change the font size!