2

I've been looking into how \documentclass and friends are actually defined, and I noticed something peculiar which I'd like to understand.

Let's say I have \documentclass[11pt, titlepage]{article}. From what I gather, \documentclass ultimately expands to \@fileswith@ptions\@clsextension, which is defined thus:

  \@ifnextchar[
  {\@fileswith@pti@ns#1[{#2}]#3}%
  {\@fileswith@pti@ns#1[{#2}]#3[]}}

I gather from that, that the optional parameters 11pt, titlepage are now put in braces. From simple experiments of the form \edef\foo{{{\bar}}}, it seems that those braces are not magically removed somehow, i.e. they seem to be preserved in further expansions, even when using \edef - i.e. from all I can gather the expansion is:

\@fileswith@pti@ns\@clsextension[{11pt,titlepage}]article[].

  • First confusion: \@fileswith@pti@ns contains the line \xdef\@classoptionslist{\zap@space#2 \@empty}, which it seems to me is intended to remove the space between 11pt, and titlepage. But since \@fileswith@ptions put the optional parameters in braces, that shouldn't have any effect. In deed, trying \zap@space {a b} c \@empty yields a bc rather than abc.
  • Furthermore, \@fileswith@pti@ns ultimately does \@onefilewithoptions#3[{#2}][{#4}]#1 - meaning it puts an additional pair of braces around the optional parameters, yielding

    \@onefilewithoptions article[{{11pt, titlepage}}][{}]\@clsextension. So now we have two spurious pairs of braces around the package parameters.

But all the further processing of the parameters is (ultimately) based on for-loops and \@ins, which split at commas (using parameter strings with comma-delimiters). So it seems to me that the braces that are introduced by \@fileswith@ptions and \@fileswith@pti@ns have to either be removed somewhere, or in some places they are ignored...? Can someone explain TeXs magic behavior here?

4

TeX drops the braces from a macro argument if the entire argument is in a brace group so in

\frac{1}{2}

#1 is 1 not {1}

But this also applies to delimited arguments after

\def\foo[#1]#2{....}

then in \foo[{abc}]{xyz} #1 is abc and #2 is xyz both brace groups are dropped.

the code you quote does [{#2}] in case #2 contains a ] in which case the delimited argument matching would fail, but the braces surround the entire argument so are never passed on to the macro being called.

| improve this answer | |

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.