Paths for Referring to an Item in the Module Tree
To show Rust where to find an item in a module tree, we use a path in the same way we use a path when navigating a filesystem. If we want to call a function, we need to know its path.
A path can take two forms:
- An absolute path starts from a crate root by using a crate name or a
literal
crate. - A relative path starts from the current module and uses
self,super, or an identifier in the current module.
Both absolute and relative paths are followed by one or more identifiers
separated by double colons (::).
Let’s return to the example in Listing 7-1. How do we call the
add_to_waitlist function? This is the same as asking, what’s the path of the
add_to_waitlist function? In Listing 7-3, we simplified our code a bit by
removing some of the modules and functions. We’ll show two ways to call the
add_to_waitlist function from a new function eat_at_restaurant defined in
the crate root. The eat_at_restaurant function is part of our library crate’s
public API, so we mark it with the pub keyword. In the ”Exposing Paths with
the pub Keyword” section, we’ll go into more detail
about pub. Note that this example won’t compile just yet; we’ll explain why
in a bit.
Filename: src/lib.rs
mod front_of_house {
mod hosting {
fn add_to_waitlist() {}
}
}
pub fn eat_at_restaurant() {
// Absolute path
crate::front_of_house::hosting::add_to_waitlist();
// Relative path
front_of_house::hosting::add_to_waitlist();
}
Listing 7-3: Calling the add_to_waitlist function using
absolute and relative paths
The first time we call the add_to_waitlist function in eat_at_restaurant,
we use an absolute path. The add_to_waitlist function is defined in the same
crate as eat_at_restaurant, which means we can use the crate keyword to
start an absolute path.
After crate, we include each of the successive modules until we make our way
to add_to_waitlist. You can imagine a filesystem with the same structure, and
we’d specify the path /front_of_house/hosting/add_to_waitlist to run the
add_to_waitlist program; using the crate name to start from the crate root
is like using / to start from the filesystem root in your shell.
The second time we call add_to_waitlist in eat_at_restaurant, we use a
relative path. The path starts with front_of_house, the name of the module
defined at the same level of the module tree as eat_at_restaurant. Here the
filesystem equivalent would be using the path
front_of_house/hosting/add_to_waitlist. Starting with a name means that the
path is relative.
Choosing whether to use a relative or absolute path is a decision you’ll make
based on your project. The decision should depend on whether you’re more likely
to move item definition code separately from or together with the code that
uses the item. For example, if we move the front_of_house module and the
eat_at_restaurant function into a module named customer_experience, we’d
need to update the absolute path to add_to_waitlist, but the relative path
would still be valid. However, if we moved the eat_at_restaurant function
separately into a module named dining, the absolute path to the
add_to_waitlist call would stay the same, but the relative path would need to
be updated. Our preference is to specify absolute paths because it’s more
likely to move code definitions and item calls independently of each other.
Let’s try to compile Listing 7-3 and find out why it won’t compile yet! The error we get is shown in Listing 7-4.
$ cargo build
Compiling restaurant v0.1.0 (file:///projects/restaurant)
error[E0603]: module `hosting` is private
--> src/lib.rs:9:28
|
9 | crate::front_of_house::hosting::add_to_waitlist();
| ^^^^^^^ private module
|
note: the module `hosting` is defined here
--> src/lib.rs:2:5
|
2 | mod hosting {
| ^^^^^^^^^^^
error[E0603]: module `hosting` is private
--> src/lib.rs:12:21
|
12 | front_of_house::hosting::add_to_waitlist();
| ^^^^^^^ private module
|
note: the module `hosting` is defined here
--> src/lib.rs:2:5
|
2 | mod hosting {
| ^^^^^^^^^^^
error: aborting due to 2 previous errors
For more information about this error, try `rustc --explain E0603`.
error: could not compile `restaurant`
To learn more, run the command again with --verbose.
Listing 7-4: Compiler errors from building the code in Listing 7-3
The error messages say that module hosting is private. In other words, we
have the correct paths for the hosting module and the add_to_waitlist
function, but Rust won’t let us use them because it doesn’t have access to the
private sections.
Modules aren’t useful only for organizing your code. They also define Rust’s privacy boundary: the line that encapsulates the implementation details external code isn’t allowed to know about, call, or rely on. So, if you want to make an item like a function or struct private, you put it in a module.
The way privacy works in Rust is that all items (functions, methods, structs, enums, modules, and constants) are private by default. Items in a parent module can’t use the private items inside child modules, but items in child modules can use the items in their ancestor modules. The reason is that child modules wrap and hide their implementation details, but the child modules can see the context in which they’re defined. To continue with the restaurant metaphor, think of the privacy rules as being like the back office of a restaurant: what goes on in there is private to restaurant customers, but office managers can see and do everything in the restaurant in which they operate.
Rust chose to have the module system function this way so that hiding inner
implementation details is the default. That way, you know which parts of the
inner code you can change without breaking outer code. But you can expose inner
parts of child modules' code to outer ancestor modules by using the pub
keyword to make an item public.
Exposing Paths with the pub Keyword
Let’s return to the error in Listing 7-4 that told us the hosting module is
private. We want the eat_at_restaurant function in the parent module to have
access to the add_to_waitlist function in the child module, so we mark the
hosting module with the pub keyword, as shown in Listing 7-5.
Filename: src/lib.rs
mod front_of_house {
pub mod hosting {
fn add_to_waitlist() {}
}
}
pub fn eat_at_restaurant() {
// Absolute path
crate::front_of_house::hosting::add_to_waitlist();
// Relative path
front_of_house::hosting::add_to_waitlist();
}
Listing 7-5: Declaring the hosting module as pub to
use it from eat_at_restaurant
Unfortunately, the code in Listing 7-5 still results in an error, as shown in Listing 7-6.
$ cargo build
Compiling restaurant v0.1.0 (file:///projects/restaurant)
error[E0603]: function `add_to_waitlist` is private
--> src/lib.rs:9:37
|
9 | crate::front_of_house::hosting::add_to_waitlist();
| ^^^^^^^^^^^^^^^ private function
|
note: the function `add_to_waitlist` is defined here
--> src/lib.rs:3:9
|
3 | fn add_to_waitlist() {}
| ^^^^^^^^^^^^^^^^^^^^
error[E0603]: function `add_to_waitlist` is private
--> src/lib.rs:12:30
|
12 | front_of_house::hosting::add_to_waitlist();
| ^^^^^^^^^^^^^^^ private function
|
note: the function `add_to_waitlist` is defined here
--> src/lib.rs:3:9
|
3 | fn add_to_waitlist() {}
| ^^^^^^^^^^^^^^^^^^^^
error: aborting due to 2 previous errors
For more information about this error, try `rustc --explain E0603`.
error: could not compile `restaurant`
To learn more, run the command again with --verbose.
Listing 7-6: Compiler errors from building the code in Listing 7-5
What happened? Adding the pub keyword in front of mod hosting makes the
module public. With this change, if we can access front_of_house, we can
access hosting. But the contents of hosting are still private; making the
module public doesn’t make its contents public. The pub keyword on a module
only lets code in its ancestor modules refer to it.
The errors in Listing 7-6 say that the add_to_waitlist function is private.
The privacy rules apply to structs, enums, functions, and methods as well as
modules.
Let’s also make the add_to_waitlist function public by adding the pub
keyword before its definition, as in Listing 7-7.
Filename: src/lib.rs
mod front_of_house {
pub mod hosting {
pub fn add_to_waitlist() {}
}
}
pub fn eat_at_restaurant() {
// Absolute path
crate::front_of_house::hosting::add_to_waitlist();
// Relative path
front_of_house::hosting::add_to_waitlist();
}
Listing 7-7: Adding the pub keyword to mod hosting
and fn add_to_waitlist lets us call the function from
eat_at_restaurant
Now the code will compile! Let’s look at the absolute and the relative path and
double-check why adding the pub keyword lets us use these paths in
add_to_waitlist with respect to the privacy rules.
In the absolute path, we start with crate, the root of our crate’s module
tree. Then the front_of_house module is defined in the crate root. The
front_of_house module isn’t public, but because the eat_at_restaurant
function is defined in the same module as front_of_house (that is,
eat_at_restaurant and front_of_house are siblings), we can refer to
front_of_house from eat_at_restaurant. Next is the hosting module marked
with pub. We can access the parent module of hosting, so we can access
hosting. Finally, the add_to_waitlist function is marked with pub and we
can access its parent module, so this function call works!
In the relative path, the logic is the same as the absolute path except for the
first step: rather than starting from the crate root, the path starts from
front_of_house. The front_of_house module is defined within the same module
as eat_at_restaurant, so the relative path starting from the module in which
eat_at_restaurant is defined works. Then, because hosting and
add_to_waitlist are marked with pub, the rest of the path works, and this
function call is valid!
Starting Relative Paths with super
We can also construct relative paths that begin in the parent module by using
super at the start of the path. This is like starting a filesystem path with
the .. syntax. Why would we want to do this?
Consider the code in Listing 7-8 that models the situation in which a chef
fixes an incorrect order and personally brings it out to the customer. The
function fix_incorrect_order calls the function serve_order by specifying
the path to serve_order starting with super:
Filename: src/lib.rs
fn serve_order() {}
mod back_of_house {
fn fix_incorrect_order() {
cook_order();
super::serve_order();
}
fn cook_order() {}
}
Listing 7-8: Calling a function using a relative path
starting with super
The fix_incorrect_order function is in the back_of_house module, so we can
use super to go to the parent module of back_of_house, which in this case
is crate, the root. From there, we look for serve_order and find it.
Success! We think the back_of_house module and the serve_order function are
likely to stay in the same relationship to each other and get moved together
should we decide to reorganize the crate’s module tree. Therefore, we used
super so we’ll have fewer places to update code in the future if this code
gets moved to a different module.
Making Structs and Enums Public
We can also use pub to designate structs and enums as public, but there are a
few extra details. If we use pub before a struct definition, we make the
struct public, but the struct’s fields will still be private. We can make each
field public or not on a case-by-case basis. In Listing 7-9, we’ve defined a
public back_of_house::Breakfast struct with a public toast field but a
private seasonal_fruit field. This models the case in a restaurant where the
customer can pick the type of bread that comes with a meal, but the chef
decides which fruit accompanies the meal based on what’s in season and in
stock. The available fruit changes quickly, so customers can’t choose the fruit
or even see which fruit they’ll get.
Filename: src/lib.rs
mod back_of_house {
pub struct Breakfast {
pub toast: String,
seasonal_fruit: String,
}
impl Breakfast {
pub fn summer(toast: &str) -> Breakfast {
Breakfast {
toast: String::from(toast),
seasonal_fruit: String::from("peaches"),
}
}
}
}
pub fn eat_at_restaurant() {
// Order a breakfast in the summer with Rye toast
let mut meal = back_of_house::Breakfast::summer("Rye");
// Change our mind about what bread we'd like
meal.toast = String::from("Wheat");
println!("I'd like {} toast please", meal.toast);
// The next line won't compile if we uncomment it; we're not allowed
// to see or modify the seasonal fruit that comes with the meal
// meal.seasonal_fruit = String::from("blueberries");
}
Listing 7-9: A struct with some public fields and some private fields
Because the toast field in the back_of_house::Breakfast struct is public,
in eat_at_restaurant we can write and read to the toast field using dot
notation. Notice that we can’t use the seasonal_fruit field in
eat_at_restaurant because seasonal_fruit is private. Try uncommenting the
line modifying the seasonal_fruit field value to see what error you get!
Also, note that because back_of_house::Breakfast has a private field, the
struct needs to provide a public associated function that constructs an
instance of Breakfast (we’ve named it summer here). If Breakfast didn’t
have such a function, we couldn’t create an instance of Breakfast in
eat_at_restaurant because we couldn’t set the value of the private
seasonal_fruit field in eat_at_restaurant.
In contrast, if we make an enum public, all of its variants are then public. We
only need the pub before the enum keyword, as shown in Listing 7-10.
Filename: src/lib.rs
mod back_of_house {
pub enum Appetizer {
Soup,
Salad,
}
}
pub fn eat_at_restaurant() {
let order1 = back_of_house::Appetizer::Soup;
let order2 = back_of_house::Appetizer::Salad;
}
Listing 7-10: Designating an enum as public makes all its variants public
Because we made the Appetizer enum public, we can use the Soup and Salad
variants in eat_at_restaurant. Enums aren’t very useful unless their variants
are public; it would be annoying to have to annotate all enum variants with
pub in every case, so the default for enum variants is to be public. Structs
are often useful without their fields being public, so struct fields follow the
general rule of everything being private by default unless annotated with pub.
There’s one more situation involving pub that we haven’t covered, and that is
our last module system feature: the use keyword. We’ll cover use by itself
first, and then we’ll show how to combine pub and use.