Is there an \iftex4ht command, and if not, how should it be defined?

Question

Background: I'm using TikZ to generate a load of pictures that I want to put on the web as SVGs. So I'm using the pgfsys-tex4ht.def driver and it's working very well. My gut feeling is that the SVG output from TikZ/pgf is "experimental" so while designing the pictures, I'm not using a special driver but just generating the PDFs via pdflatex. Also, I want PDFs as well as SVGs at the end.

So rather than having to comment out the line

\def\pgfsysdriver{pgfsys-tex4ht.def}

each time, I'd rather have a switch that loaded it if tex4ht was loaded; that is, I already specify the output when I decide whether to run pdflatex or htlatex - I don't want to have to specify it twice. So, my question:

Is there an \iftex4ht somewhat akin to the \ifpdf provided by ifpdf.sty?
If not, what would be the "right" way to test for the presence of tex4ht?

Answer 1

There is always this:

\@ifpackageloaded{tex4ht}{LOADED}{NOT LOADED}

Suitable for other packages of interest as well, of course. (Usual caveats about @ character apply.)

Answer 2

A conditional named \iftex4ht is impossible to define without side effects. However using directly \@ifpackageloaded{tex4ht} has a big limitation, because it's allowed only in the preamble.

There are a few strategies to circumvent the problem: all use \@ifpackageloaded in the preamble to define something that one will be able to use also in the body of the document.

1. Primitive conditional syntax

   \makeatletter
   \@ifpackageloaded{tex4ht}
     {\let\iftexforht\iftrue}
     {\let\iftexforht\iffalse}
   \makeatother
   

   and use \iftexforht just like any of the primitive conditionals.

2. LaTeX pseudoconditional

   \makeatletter
   \@ifpackageloaded{tex4ht}
     {\let\iftexforhtTF\@firstoftwo}
     {\let\iftexforhtTF\@secondoftwo}
   \makeatother
   

   to be used like

   \iftexforhtTF{<code for TeX4ht>}{<code when TeX4ht isn't used>}
   

3. Almost like a primitive conditional

   \makeatletter
   \edef\texforht{TT\noexpand\fi
     \@ifpackageloaded{tex4ht}
       {\noexpand\iftrue}
       {\noexpand\iffalse}}
   \makeatother
   

   to be used as

   \if\texforht
     <code for TeX4ht>
   \else
     <code when TeX4ht isn't used>
   \fi
   

Which one to prefer is a matter of taste. Both the first and third methods allow conditional nesting.

---

Prompted by extra \fi error when using \localtableofcontents inside an \if...\else..\fi, there is another reason for using the LaTeX conditional style (case 2 in the list above).

When used in the preamble, a construction such as

\iftexforht
  <code for TeX4ht case>
\else
  <code for non TeX4ht case>
\fi

is usually good and practical. However when used in the document body, you can incur in problems. For instance, if you have

\iftexforht\else\localtableofcontents\fi

as in the linked question, this won't work and throw a mysterious error message, because of how \localtableofcontents is defined. Instead,

\iftexforhtTF{}{\localtableofcontents}

would have worked properly.

The \iftexforhtTF macro can be defined either with the code above or, with Nasser's idea,

\makeatletter
\ifdefined\HCode
   \let\iftexforhtTF\@firstoftwo
\else
   \let\iftexforhtTF\@secondoftwo
\fi
\makeatother

Credit to karlkoeller for spotting the problem in the linked question.

Answer 3

The following can be used. An advantage to this is that it does not need to be defined before. Hence it can be used in the preamble and the body

\ifdefined\HCode
  %htlatex code here
\else
   %pdflatex code here
\fi

some examples

Example 1

\documentclass[]{article}
\begin{document}

\ifdefined\HCode
  This show in tex4ht only
\else
  This shows in PDF only
\fi

\end{document}    

Running the above as pdflatex foo.tex produces This shows in PDF only and running it using htlatex foo.tex produces web page with This show in tex4ht only

Example 2

It can also be used in preamble

\documentclass[]{article}

\ifdefined\HCode
\else
   \usepackage{pdfpages}   %MAKE SURE YOU DO NOT USE THIS WITH htlatex
\fi

\begin{document}
% ....
\end{document}

Example 3

It can be used to load different class options also

\ifdefined\HCode
  \documentclass[]{article}
\else
  \documentclass[twocolumn]{article}  
\fi

\usepackage{lipsum}

\begin{document}    
\lipsum[1-20]    
\end{document}

Answer 4

Based on Harald's answer you can define your \iftex4ht as follows.

\documentclass{article}
%\usepackage{tex4ht}
\makeatletter
\@ifpackageloaded{tex4ht}{\def\iftex4ht{\iftrue}}
                         {\def\iftex4ht{\iffalse}}
\makeatother

\begin{document}
\iftex4ht TeX4ht\else no!\fi
\end{document}

OK, this answer is kind of a joke, but I wanted to point out that you can't use \newif\iftex4ht or \newcommand\iftex4ht since 4 is not letter. With \def it works (delimited macro with no arguments), but you can't nest it in other \ifs since TeX doesn't see that it's an \if.