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author | Karl Berry <karl@freefriends.org> | 2009-05-22 23:51:44 +0000 |
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committer | Karl Berry <karl@freefriends.org> | 2009-05-22 23:51:44 +0000 |
commit | 566f5207d7e3cafb0633d31277067336ccd9cca7 (patch) | |
tree | 7af53e7ac405185168bc1dc38c4e820f442b78d0 /Master/texmf-dist/doc/generic/FAQ-en/html/FAQ-cvtlatex.html | |
parent | b0beea26ffffd915bb6d9b82f2d65a0a05b7e23b (diff) |
move generic english documents out of texmf-doc
git-svn-id: svn://tug.org/texlive/trunk@13396 c570f23f-e606-0410-a88d-b1316a301751
Diffstat (limited to 'Master/texmf-dist/doc/generic/FAQ-en/html/FAQ-cvtlatex.html')
-rw-r--r-- | Master/texmf-dist/doc/generic/FAQ-en/html/FAQ-cvtlatex.html | 77 |
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diff --git a/Master/texmf-dist/doc/generic/FAQ-en/html/FAQ-cvtlatex.html b/Master/texmf-dist/doc/generic/FAQ-en/html/FAQ-cvtlatex.html new file mode 100644 index 00000000000..cdaf84cfbbf --- /dev/null +++ b/Master/texmf-dist/doc/generic/FAQ-en/html/FAQ-cvtlatex.html @@ -0,0 +1,77 @@ +<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’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 “robust” +(i.e., will not expand if subjected to LaTeX “protected +expansion”); from the Plain TeX user’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’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 (0–9: 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: +<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> |