These days I type most of simple documents (short articles, blog entries, course notes) in markdown. Markdown provides only the basic structured elements (sections, emphasis, urls, lists, footnotes, syntax highlighting, simple tables and figures) which makes it easy to transform the input into multiple output formats. Most of the time, I still want PDF output and for that, I use pandoc to convert markdown to ConTeXt. At the same time, I have the peace of mind that if I need HTML or DOC output, I’ll be able to get that easily.
For most of the last decade, I have almost exclusively used LaTeX/ConTeXt for writing all my documents. After moving to Markdown, I miss three features of TeX: separation of content and presentation; conditional inclusion of content; and including external documents. In this post, I’ll explain how to get these with Markdown.
Separation of content and presentation
TeX gives you a lot of control for creating new structural elements. Let’s take a simple example. Suppose I want to write a file name in a document. Normally, I want the filename to appear in typewriter font. In LaTeX, I could type it as
\texttt{src/hello.c}
but it is better to define a custom macro \filename and use
\filename{src/hello.c}
The advantage is two-fold. Firstly, while writing the file, I am thinking in term of content (filename) rather than presentation (typewriter font). Secondly, in the future, if I want to change how a filename is displayed (perhaps as a hyper-link to the file), all I need to do is change the definition of the macro. Markdown, with its simplistic structure, lacks the ability to define custom macros.
Conditional compilation
TeX also makes it trivial to generate multiple versions of the document from the same source. Again, lets take an example. Suppose I am writing notes for a class. Normally, I like to include a short bullet list on my lecture slides, but include a detailed description in the lecture handout. In ConTeXt I can use modes as follows (LaTeX has a similar feature using the comments package):
Feature of the solution
\startitemize[n]
\item Feature 1
\startmode[handout]
Explanation of the feature ...
\stopmode
\item Feature 2
\startmode[handout]
Explanation of the feature ...
\stopmode
\stopitemize
To generate the slides version of my lecture notes, I compile them using
context --mode=slides --result=slides <filename>
This version just contains the bullet list. Since the handout mode is not set, the content between \startmode[handout] ... \stopmode is omitted.
To generate the handout version of my lecture notes, I compile them using
context --mode=handout --result=slides <filename>
Since the handout mode is set, the content between \startmode[handout] ... \stopmode is included
Such a conditional compilation is extremely useful to keep the slides and handouts in sync. Again, markdown with its simplistic feature set, lacks the ability of conditional compilation. Neither does Pandoc add this feature.
Including external documents
TeX makes it easy to include external documents. This is really important when you want to include source code in your documents. I teach an introductory programming class, and want to make sure that the example code included in my notes is correct. I write the code in a separate file, write the corresponding test files to ensure that the code works correctly, and then include it in my notes using
\typeJAVAfile[src/FactoryExample.java]
which gives me syntax highlighted source code. Pandoc does generate syntax highlighted source code, but does not provide any means to include external source code. So, I have to copy paste the code from the actual source file to the markdown document, but that is an error-prone process.
If I only cared about PDF output (via LaTeX/ConTeXt backend), I could simply use the same TeX macros in the markdown document. Pandoc passes the TeX macros unchanged to the LaTeX/ConTeXt backend, so I would get a TeX document with all the bells and whistles. But, if I tried to generate HTML or DOC output, these TeX macros will be omitted, and I’d get a broken document. One of my reasons to switching to Markdown was the peace of mind that I can generate HTML or DOC output if needed. Using TeX macros in the source takes away that advantage.
So, I started looking for possible solutions and found gpp—the generic pre-processor. It is similar to the C-preprocessor (that handles the #include, #define, stuff in C/C++) but provides many configuration options. I use it with the -H option, which requires macros to be specified in an HTML-like mode:
<#include "file">
<#define MACRO|value>
Use <#MACRO>
Normally the <#...> does not appear in a document, so using gpp is safe.
See the gpp documentation for complete details. I’ll show how to get the three features that I miss from TeX:
- Separation of content and presentationWith
gppI can define new macros that denote new structural elements, e.g.,
<#define filename|`#1`>
The source is included in <#filename src/hello.c>
When I compile the document using gpp -H, I get
The source is included in `src/hello.c`
Sure, this requires more typing that simply using `...`, but that is the price that one has to pay for getting more structure. More importantly, I can define the #filename macro based on the output format:
<#define filename|`#1`>
<#ifdef HTML>
<#define filename|<code class="filename">#1</code>>
<#endif>
<#ifdef TEX>
<#define filename|\\filename{#1}>
<#endif>
The source is included in <#filename src/hello.c>
Now, if I compile the document using gpp -H -DHTML=1, I get
The source is included in <code class="filename">src/hello.c</code
and if I compile using gpp -H -DTEX=1, I get
The source is included in \filename{src/hello.c}
This ensures that the document structure is passed to the output as well.
To make it easy to manage macros, create three files, macros.gpp containing all macros, html.gpp overwriting some of the macros with HTML equivalents, and tex.gpp overwriting some of the macros with TeX equivalents. End macros.cpp file with
....
<#ifdef HTML>
<#include "html.gpp">
<#endif>
<#ifdef TEX>
<#include "tex.gpp">
<#endif>
and then preprocess the document using gpp -DTEX=1 --include macors.gpp <filename> (or -DHTML=1 for HTML output).
- Conditional compilationActually, the previous example already shows how to get conditional compilation: use the
-D command line switch and check the variable definition using #ifdef. Thus, the above example translates to:
Feature of the solution
1. Feature 1
<#ifdef HANDOUT>
Explanation of the feature ...
<#endif>
2. Feature 2
<#ifdef HANDOUT>
Explanation of the feature ...
<#endif>
When I compile without -DHANDOUT=1, I get the slides version; when I compile with -DHANDOUT-1, I get the handout version.
- Including external documentsExternal documents can be included with
#includedirective. So, I can include an external file using
~~~ {.java}
<#include "src/Factory.java">
~~~
Putting it all together
All that is needed is to run the gpp preprocessor and then pass the output to pandoc.
gpp -H <options> <filename> | pandoc -f markdown -t <format> -o <outfile>
Hide this in a wrapper script or a shell function or a Makefile, and you have a markdown processor with the important features of TeX!
Random Determinism 