\
def
, is that
there’s a general philosophy within LaTeX that the user should be
protected from himself: the user has different commands according to
whether the command to be defined exists (\
renewcommand
) or not
(\
newcommand
), and if its status proves not as the user expected,
an error is reported. A third definition command,
\
providecommand
, only defines if the target is not already
defined; LaTeX has no direct equivalent of \
def
, which ignores
the present state of the command. The final command of this sort is
\
DeclareRobustCommand
, which creates a command which is “robust”
(i.e., will not expand if subjected to LaTeX “protected
expansion”); from the Plain TeX user’s point of view,
\
DeclareRobustCommand
should be treated as a non-checking version
of \
newcommand
.
LaTeX commands are, by default, defined \
long
; an optional *
between the \
newcommand
and its (other) arguments specifies that
the command is not to be defined \
long
. The *
is
detected by a command \
@ifstar
which uses \
futurelet
to switch
between two branches, and gobbles the *
: LaTeX users are
encouraged to think of the *
as part of the command name.
LaTeX’s checks for unknown command are done by \
ifx
comparison
of a \
csname
construction with \
relax
; since the command name
argument is the desired control sequence name, this proves a little
long-winded. Since #1
is the requisite argument, we have:
\expandafter\ifx \csname\expandafter\@gobble\string#1\endcsname \relax ...
(\
@gobble
simply throws away its argument).
\newcommand\foo{...} \newcommand\foo[0]{...} \newcommand\foo[1]{...#1...} \newcommand\foo[2][boo]{...#1...#2...}
In the last case, \
foo
may be called as \
foo{goodbye}
,
which is equivalent to \
foo[boo]{goodbye}
(employing the
default value given for the first argument), or as
\
foo[hello]{goodbye}
(with an explicit first argument).
\
foo
above:
\def\foo{\futurelet\next\@r@foo} \def\@r@foo{\ifx\next[% \let\next\@x@foo \else \def\next{\@x@foo[boo]}% \fi \next } \def\@x@foo[#1]#2{...#1...#2...}
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