### Transcribing LaTeX command definitions

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 ``; 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 command 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...}