# Introduction

[Metalang99](https://github.com/hirrolot/metalang99) is a functional macro language that allows you to create useful metaprograms in pure C99. Implemented as a header-only library, Metalang99 extends the standard C preprocessor with extra programming capabilities, including recursion, algebraic data types, collections, currying, natural numbers, functional composition, debugging facilities, and more.

Having Metalang99 at our disposal, it becomes possible to extend the C programming language with custom syntax sugar, via such advanced metaprograms as [Datatype99](https://github.com/hirrolot/datatype99):

```c
#include <datatype99.h>

datatype(
    BinaryTree,
    (Leaf, int),
    (Node, BinaryTree *, int, BinaryTree *)
);

int sum(const BinaryTree *tree) {
    match(*tree) {
        of(Leaf, x) return *x;
        of(Node, lhs, x, rhs) return sum(*lhs) + *x + sum(*rhs);
    }

    return -1;
}
```

Or [Interface99](https://github.com/hirrolot/interface99):

```c
#include <interface99.h>

#include <stdio.h>

#define Shape_IFACE                      \
    vfunc( int, perim, const VSelf)      \
    vfunc(void, scale, VSelf, int factor)

interface(Shape);

typedef struct {
    int a, b;
} Rectangle;

int  Rectangle_perim(const VSelf) { /* ... */ }
void Rectangle_scale(VSelf, int factor) { /* ... */ }

impl(Shape, Rectangle);

typedef struct {
    int a, b, c;
} Triangle;

int  Triangle_perim(const VSelf) { /* ... */ }
void Triangle_scale(VSelf, int factor) { /* ... */ }

impl(Shape, Triangle);

void test(Shape shape) {
    printf("perim = %d\n", VCALL(shape, perim));
    VCALL(shape, scale, 5);
    printf("perim = %d\n", VCALL(shape, perim));
}
```

Throughout this book, you will learn the fundamentals of Metalang99, starting from the [core metalanguage](https://hirrolot.gitbook.io/metalang99/syntax-and-semantics) and ending with various [testing, debugging, and error reporting](https://hirrolot.gitbook.io/metalang99/testing-debugging-and-error-reporting) facilities. For more information, please refer to the following material:

* [The official GitHub repository](https://github.com/hirrolot/metalang99);
* [The formal specification](https://github.com/hirrolot/metalang99/blob/master/spec/spec.pdf);
* [The Metalang99 standard library documentation](https://metalang99.readthedocs.io/en/latest/);
* [The list of publications about Metalang99](https://github.com/hirrolot/metalang99#publications).

Let's start!


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