Defining Modules to Control Scope and Privacy

In this section, we’ll talk about modules and other parts of the module system, namely paths that allow you to name items; the use keyword that brings a path into scope; and the pub keyword to make items public. We’ll also discuss using the as keyword, external packages, and the glob operator. For now, let’s focus on modules!

Modules let us organize code within a crate into groups for readability and easy reuse. Modules also control the privacy of items, which is whether an item can be used by outside code (public) or whether it’s an internal implementation detail and not available for outside use (private).

As an example, let’s write a library crate that provides the functionality of a restaurant. We’ll define the signatures of functions but leave their bodies empty to concentrate on the organization of the code rather than actually implementing a restaurant in code.

In the restaurant industry, parts of a restaurant are referred to as front of house and others as back of house. Front of house is where customers are and includes hosts seating customers, servers taking orders and payment, and bartenders making drinks. Back of house includes the chefs and cooks in the kitchen, dishwashers cleaning up, and managers doing administrative work.

To structure our crate in the same way that a real restaurant works, we can organize the functions into nested modules. Create a new library named restaurant by running cargo new --lib restaurant; then put the code in Listing 7-1 into src/lib.rs to define some modules and function signatures.

Filename: src/lib.rs

mod front_of_house {
    mod hosting {
        fn add_to_waitlist() {}

        fn seat_at_table() {}
    }

    mod serving {
        fn take_order() {}

        fn serve_order() {}

        fn take_payment() {}
    }
}

Listing 7-1: A front_of_house module containing other modules that then contain functions

We define a module by starting with the mod keyword, and then specify the name of the module (in this case, front_of_house) and place curly brackets around the body of the module. Inside modules, we can have other modules, as in this case with the modules hosting and serving. Modules can also hold definitions for other items, such as structs, enums, constants, traits, or as in Listing 7-1, functions.

By using modules, we can group related definitions together and name why they’re related. Programmers using this code would have an easier time finding the definitions they want to use because they could navigate the code based on the groups rather than having to read through all the definitions. Programmers adding new functionality to this code would know where to place the code to keep the program organized.

Earlier, we mentioned that src/main.rs and src/lib.rs are called crate roots. The reason for their name is that the contents of either of these two files form a module named crate at the root of the crate’s module structure, known as the module tree.

Listing 7-2 shows the module tree for the structure in Listing 7-1.

crate
 └── front_of_house
     ├── hosting
     │   ├── add_to_waitlist
     │   └── seat_at_table
     └── serving
         ├── take_order
         ├── serve_order
         └── take_payment

Listing 7-2: The module tree for the code in Listing 7-1

This tree shows how some of the modules nest inside one another (such as hosting nests inside front_of_house). The tree also shows how some modules are siblings to each other, meaning they’re defined in the same module (hosting and serving are defined within front_of_house). To continue the family metaphor, if module A is contained inside module B, we say that module A is the child of module B, and that module B is the parent of module A. Notice that the entire module tree is rooted under the implicit module named crate.

The module tree might remind you of the filesystem’s directory tree on your computer; this is a very apt comparison! Just like directories in a filesystem, you use modules to organize your code. And just like files in a directory, we need a way to find our modules.