When you start writing programs in Rust, your code might live solely in the main function. As your code grows, you’ll eventually move functionality into other functions for reuse and better organization. By splitting your code into smaller chunks, you make each chunk easier to understand on its own. But what happens if you have too many functions? Rust has a module system that enables the reuse of code in an organized fashion.

In the same way that you extract lines of code into a function, you can extract functions (and other code, like structs and enums) into different modules. A module is a namespace that contains definitions of functions or types, and you can choose whether those definitions are visible outside their module (public) or not (private). Here’s an overview of how modules work:

• The mod keyword declares a new module. Code within the module appears either immediately following this declaration within curly brackets or in another file.
• By default, functions, types, constants, and modules are private. The pub keyword makes an item public and therefore visible outside its namespace.
• The use keyword brings modules, or the definitions inside modules, into scope so it’s easier to refer to them.

We’ll look at each of these parts to see how they fit into the whole.