At several places in this FAQ, questions are answered in terms of how to program a LaTeX macro. Sometimes, these macros might also help users of Plain TeX or other packages; this answer attempts to provide a rough-and-ready guide to transcribing such macro definitions for use in other packages.
The reason LaTeX has commands that replace \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).
The arguments of a LaTeX command are specified by two optional arguments to the defining command: a count of arguments (0–9: if the count is 0, the optional count argument may be omitted), and a default value for the first argument, if the defined command’s first argument is to be optional. So:
\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).
Coding of commands with optional arguments is exemplified by the
coding of the last \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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