Modules

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 out into other functions, both for re-use and for better organization. By splitting your code up into smaller chunks, each chunk is easier to understand on its own. But what happens if you find yourself with too many functions? Rust has a module system that handles the problem of wanting to re-use code while keeping your code organized.

In the same way that you extract lines of code into a function, you can extract functions (and other code like structs and enums too) 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:

• You declare a new module with the keyword mod
• By default, everything is set as private (including modules). You can use the pub keyword to make a module public and therefore visible outside of its namespace.
• The use keyword allows you to bring modules, or the definitions inside modules, into scope so that it’s easier to refer to them.

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