Languages like C have a preprocessor. The preprocessor preprocesses the source code before compiling it. The C preprocessor, copy-pastes code from other files into the current file (#includes), erases code (#if if the condition is false), and expands macros (e.g. you have #define MAX(x, y) ((x) < (y) ? (y) : (x)), it replaces every use of that macro with the definition of that macro: a += a * MAX(b, c)→a += a * ((b) < © ? © : (b)). There are also general-purpose preprocessors (or macro processors), that are not tied to a specific programming language however. m4 is one of them and GNU autotools make extensive use of them to generate their configuration and make files. What preprocessors allow one to do is write a template, and then generate a result, based on your needs.

So I am going to explain the concept of macro preprocessors to you — m5.awk is a macro preprocessor, so is m4 so is GPP so is C’s CPP, so is my Ekipp and so is my AllocPP.pl. They all work like this:

1- Frank Frankis (hereby referred to as FF) creates a file called my-file.randext — randext here meaning that macro prerpcoessors work on all kind of files, be it a C code, a Python code, an HTML template, your tax files, your notes to your GF, your angry letter to your boss, etc;

2- There are dozens and dozens of uses for a macro preprocessors, but let’s say FF wants to obliterate two birds with one sniper shot, he wishes to write a manual for his new Car XAshtray ™, in HTML and Markdown, but he wants it to be contained within one single file – and he wishes to reuse certain fragments of text, certain HTML tags, certain Markdown syntaxes, to be ‘reusable’, just like a ‘function’ is reusable piece of code in an imperative language (but not a functional language!) or how a template works in C++, etc.

3- FF enters the file with his favorite text editor. He defines these ‘functions’, which are the ‘macro’ part of a macro preprocessor. Think what ‘macro’ means. It means ‘big picture’ basically. I think the better term here is ‘meta’. These two words have a close relationship in the English language, don’t they?

Now let’s see what macro preprocessor FF uses. Since GPP is really similar in syntax to C’s preprocessor (at least with default settings) let’s use GPP. I would use my Ekipp here but I honestly have forgotten the syntax (believe it or not, creating a language does not mean you are good at it).

#ifdef __HTML__
#define TITLE <h1>My Car XAshtray Manual</h1>
#define SUBTITLE <h5>Throw your ash on me</h5>
#define BOLDEN(text) <b>text</b>
#elif __MARKDOWN__
#define TITLE  \# My Car XAhtray Manual
#define SUBTITLE \#\#\#\#\# Throw your ash on me
#define BOLDEN(text) **text**
#else
#error "Must define a target language"
#endif

FF keeps defining these. Now comes writing the actual manual.

Keep in mind that GPP stands for ‘Generic Preprocessor’, it’s a text macro prerpcoessor and not a language preprocessor like CPP (C’s preprocessor) is. m4 and Ekipp are like that. My AllocPP.pl is a language preprocessor, it preprocesses C. So now, this means FF can now freely treat my-file.randext as a text file, with a ‘language’, the Macro Preprocessor language, defining the output (I’ll talk about what I mean by ‘output’ soon).

So he writes his manual:

TITLE
SUBTITLE

Hello! Are you okay? Why did you buy this ashtray? BOLDEN(ARE YOU OKAY?). In this manual I will teach you how to use my ashtray...
...

Replace the last ellipse with rest of the text, the Car Ashtray manual.

Now, FF needs to ‘preprocess’ this text file. This is, well, the ‘preprocessing’ part of a macro preprocessor. It’s like compiling a program with a compiler, except you are compiling text-to-text instead of text-to-binary.

gpp -D__MARKDOWN__ my-file.randext > my-manual.md
``
But what happened at `-D__MARKDOWN__`? I think you have already guessed. In the 'program' we asserted if '__MARKDOWN__' is 'defined', then then define those macros as Markdown, else HTML. We can also define a macro with a value:

gpp -DMyMacro=MyValue my-file.randext > my-manual.md

Now, GPP has more built-in macros like `#ifdef`, they are called 'meta macros` (as opposted to the macros you yourself define). There's `#include` which includes a file. There `#exec` which executes a shell command. Etc etc.

Now, you can read more about GPP on its Github. I was in touch with its maintainer, Tristan Miller, very recently when I showed him my Ekipp. He has made a new version of GPP so don't install it fro