I was trying to make my own customization letter class and searching for the definition of the \toname command, I found this.
A triple at several times, but I wasn't able to find information so I decided to ask.
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2 Answers
When you do
\begin{letter}{
M. Y. Friend \\
42 Some Rd \\
Someplace
}
LaTeX does
\@processto{\leavevmode\ignorespaces #1}}
which in the example case becomes
\@processto{\leavevmode\ignorespaces M. Y. Friend \\ 42 Some Rd \\ Someplace }
This will first execute
\@xproc \leavevmode\ignorespaces M. Y. Friend \\ 42 Some Rd \\ Someplace \\@@@
According to the definition of \@xproc, LaTeX will take as #1 everything up to the first \\ and #2 everything up to \\@@@ and so it will do
\def\toname{\leavevmode\ignorespaces M. Y. Friend }
\def\toadddress{ 42 Some Rd \\ Someplace \\}
Suppose instead you do
\begin{letter}{M. Y. Friend}
The same process will lead to the call
\@xproc \leavevmode\ignorespaces M. Y. Friend \\@@@
In this case #1 is again everything up to the first (and unique) \\, and #2 is everything from \\ (excluded) up to @@@, so it's empty.
The interesting case is therefore when \toaddress is not empty: a call to \@yproc is necessary in order to remove the trailing \\ and indeed the call is
\@yproc #1@@@
which will become
\@yproc \leavevmode\ignorespaces M. Y. Friend \\ 42 Some Rd \\ Someplace @@@
so it will end up in
\def\toaddress{ 42 Some Rd \\ Someplace }
Note that leading and trailing spaces are not omitted. They will “disappear” when \opening is executed, because \toname and \toaddress are typeset in the scope of \raggedright, precisely (line 192)
{\raggedright \toname \expandafter\\\toaddress \par}%
so in the example case we'll get
{\raggedright \leavevmode\ignorespaces M. Y. Friend \\ 42 Some Rd \\ Someplace \par}
Since \\ also does \par, in this context, trailing spaces are suppressed by the implicit \unskip.
Why the strange \expandafter? Because a user might like some separation between the addressee's name and the address, so typing in something like
\begin{letter}{
M. Y. Friend \\[1ex]
42 Some Rd \\
Someplace
}
Apply the same reasoning as above to see that in this case LaTeX does
\def\toaddress{[1ex] 42 Some Rd \\ Someplace }
and without the \expandafter the following \\ would not see [1ex].
The documentation indeed says
A better fix would be to do a proper parsing but…
Look at the next line where \@xproc is defined.
\long\def\@xproc #1\\#2@@@{\def\toname{#1}\def\toaddress{#2}}
\long\def\@yproc #1\\#2@@@{\def\toaddress{#2}}
There are #1 separated by \\ and #2 separated by @@@. So @@@ is simply separetor of the second parameter. The usage of \@xproc text\\@@@ (used in \@processto) does following: if text includes \\ then #2 is nonempty because #2 includes the rest of text followed by \\. If text does not include \\ then #2 is empty because #1 is whole text and we see nothing between \\ and @@@. This behavior can be broken if someone write a text including \\@@@ but macro programmer supposed that this has less probability.
