summaryrefslogtreecommitdiff
path: root/Master/texmf-dist/doc/generic/FAQ-en/html/FAQ-cvtlatex.html
blob: cdaf84cfbbf00de39421930e45d6bdabce6db58f (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
<head>
<title>UK TeX FAQ -- question label cvtlatex</title>
</head><body>
<h3>Transcribing LaTeX command definitions</h3>
<p/>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.
<p/>The reason LaTeX has commands that replace <code>\</code><code>def</code>, is that
there&rsquo;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 (<code>\</code><code>renewcommand</code>) or not
(<code>\</code><code>newcommand</code>), and if its status proves not as the user expected,
an error is reported.  A third definition command,
<code>\</code><code>providecommand</code>, only defines if the target is not already
defined; LaTeX has no direct equivalent of <code>\</code><code>def</code>, which ignores
the present state of the command.  The final command of this sort is
<code>\</code><code>DeclareRobustCommand</code>, which creates a command which is &ldquo;robust&rdquo;
(i.e., will not expand if subjected to LaTeX &ldquo;protected
expansion&rdquo;); from the Plain TeX user&rsquo;s point of view,
<code>\</code><code>DeclareRobustCommand</code> should be treated as a non-checking version
of <code>\</code><code>newcommand</code>.
<p/>LaTeX commands are, by default, defined <code>\</code><code>long</code>; an optional <code>*</code>
between the <code>\</code><code>newcommand</code> and its (other) arguments specifies that
the command is <em>not</em> to be defined <code>\</code><code>long</code>.  The <code>*</code> is
detected by a command <code>\</code><code>@ifstar</code> which uses <code>\</code><code>futurelet</code> to switch
between two branches, and gobbles the <code>*</code>: LaTeX users are
encouraged to think of the <code>*</code> as part of the command name.
<p/>LaTeX&rsquo;s checks for unknown command are done by <code>\</code><code>ifx</code> comparison
of a <code>\</code><code>csname</code> construction with <code>\</code><code>relax</code>; since the command name
argument is the desired control sequence name, this proves a little
long-winded.  Since <code>#1</code> is the requisite argument, we have:
<blockquote>

<pre>
\expandafter\ifx
  \csname\expandafter\@gobble\string#1\endcsname
  \relax
    ...
</pre>
</blockquote><p>
(<code>\</code><code>@gobble</code> simply throws away its argument).
<p/>The arguments of a LaTeX command are specified by two optional
arguments to the defining command: a count of arguments (0&ndash;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&rsquo;s first argument
is to be optional.  So:
<blockquote>
<pre>
\newcommand\foo{...}
\newcommand\foo[0]{...}
\newcommand\foo[1]{...#1...}
\newcommand\foo[2][boo]{...#1...#2...}
</pre>
</blockquote><p>
In the last case, <code>\</code><code>foo</code> may be called as <code>\</code><code>foo{goodbye}</code>,
which is equivalent to <code>\</code><code>foo[boo]{goodbye}</code> (employing the
default value given for the first argument), or as
<code>\</code><code>foo[hello]{goodbye}</code> (with an explicit first argument).
<p/>Coding of commands with optional arguments is exemplified by the
coding of the last <code>\</code><code>foo</code> above:
<blockquote>
<pre>
\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...}
</pre>
</blockquote><p>
<p/><p>This question on the Web: <a href="http://www.tex.ac.uk/cgi-bin/texfaq2html?label=cvtlatex">http://www.tex.ac.uk/cgi-bin/texfaq2html?label=cvtlatex</a>
</body>