I have seen packages using
\def\afterelse#1\else#2\fi{\fi#1}
which TeX seems happy to accept. But on taking the first branch, isn't TeX expecting \else
instead of \fi
? That is, isn't the following the correct syntax for the first fork? This too is accepted by TeX.
\def\afterelse#1\else#2\fi{\else#1}
EDIT
Thanks to Egreg. Tracing \if
s shows what is happening.
\def\loggingallstuff{%
\tracinggroups\@ne\tracingifs\@ne\loggingall\tracingassigns\@ne
}
\def\afterelse#1\else#2\fi{\fi#1}
{\loggingallstuff
%\edef\x{\iftrue\afterelse T\else F\fi}
\edef\x{\iftrue T\else F\fi}
% \edef\x{\iffalse T\else F\fi}
}
% \iftrue
{\edef}
{\iftrue: (level 1) entered on line 23}
{true}
{\else: \iftrue (level 1) entered on line 23}
{\fi: \iftrue (level 1) entered on line 23}
{changing \x=undefined}
{into \x=macro:->T}
% \iffalse:
{\edef}
{\iffalse: (level 1) entered on line 24}
{false}
{\else: \iffalse (level 1) entered on line 24}
{\fi: \iffalse (level 1) entered on line 24}
{changing \x=undefined}
{into \x=macro:->F}
Here is one case I find interesting:
\documentclass{article}
\usepackage{pgfkeys}
\makeatletter
\def\pgfkeysafterfi#1\fi{\fi#1}
%\def\pgfkeysafterelsei#1\else#2\fi{\fi#1}
\def\pgfkeysafterelsei#1\else#2\fi{\else#1} % wrong but gives no error.
\def\pgfkeysifkeydef#1#2#3{%
\ifcsname pgfk@#1/.@cmd\endcsname
\pgfkeysafterelsei{#2}\else\pgfkeysafterfi{#3}\fi
}
\pgfkeys{%
/handlers/.if definable/.code 2 args={
\pgfkeysifkeydef{\pgfkeyscurrentpath}{%
\@latexerr{Oops, key '\pgfkeyscurrentpath' already exists}
{You aren't in trouble here!}%
}{%
\pgfkeys{\pgfkeyscurrentpath/.default=#1,\pgfkeyscurrentpath/.code=#2}%
}%
}%
}
\pgfkeys{
/fam/.is family,/fam/.cd,
keya/.if definable=\relax\@empty,
keya/.if definable=\@empty{\def\x##1{##1*#1}},
}
\makeatother
\begin{document}
x
\end{document}
\iftrue\else\or\fi
is legal and writes{\or: \iftrue (level 1) ...}
to the log when tracing ifs.\iffalse\def\x{T}\else\def\x{F}\or\fi
givesextra \or
.\def\foo#1{\ifx?#1abc\else def\fi}
(meant to giveabc
when#1
is?
anddef
otherwise). Giving it?\else
as an argument will produce no output. I've had somewhat similar situations withinl3fp
.