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. To call a function, we need to know its path.
A path can take two forms:
- An absolute path is the full path starting from a crate root; for code
from an external crate, the absolute path begins with the crate name, and for
code from the current crate, it starts with the 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 (::
).
Returning to Listing 7-1, say we want to call the add_to_waitlist
function.
This is the same as asking: what’s the path of the add_to_waitlist
function?
Listing 7-3 contains Listing 7-1 with some of the modules and functions
removed.
We’ll show two ways to call the add_to_waitlist
function from a new function
eat_at_restaurant
defined in the crate root. These paths are correct, but
there’s another problem remaining that will prevent this example from compiling
as-is. We’ll explain why in a bit.
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
.
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();
}
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. We then include each of the successive modules until we
make our way to add_to_waitlist
. You can imagine a filesystem with the same
structure: 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 module 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, and depends 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 in general
is to specify absolute paths because it’s more likely we’ll want 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 {
| ^^^^^^^^^^^
For more information about this error, try `rustc --explain E0603`.
error: could not compile `restaurant` due to 2 previous errors
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. In Rust, all items (functions, methods, structs, enums,
modules, and constants) are private to parent modules by default. If you want
to make an item like a function or struct private, you put it in a module.
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. This is because child modules wrap and hide their implementation details, but the child modules can see the context in which they’re defined. To continue with our 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 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. However, Rust does give
you the option to 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();
}
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() {}
| ^^^^^^^^^^^^^^^^^^^^
For more information about this error, try `rustc --explain E0603`.
error: could not compile `restaurant` due to 2 previous errors
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, not access its inner code.
Because modules are containers, there’s not much we can do by only making the
module public; we need to go further and choose to make one or more of the
items within the module public as well.
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();
}
Now the code will compile! To see why adding the pub
keyword lets us use
these paths in add_to_waitlist
with respect to the privacy rules, let’s look
at the absolute and the relative paths.
In the absolute path, we start with crate
, the root of our crate’s module
tree. The front_of_house
module is defined in the crate root. While
front_of_house
isn’t public, 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!
If you plan on sharing your library crate so other projects can use your code, your public API is your contract with users of your crate that determines how they can interact with your code. There are many considerations around managing changes to your public API to make it easier for people to depend on your crate. These considerations are out of the scope of this book; if you’re interested in this topic, see The Rust API Guidelines.
Best Practices for Packages with a Binary and a Library
We mentioned a package can contain both a src/main.rs binary crate root as well as a src/lib.rs library crate root, and both crates will have the package name by default. Typically, packages with this pattern of containing both a library and a binary crate will have just enough code in the binary crate to start an executable that calls code with the library crate. This lets other projects benefit from the most functionality that the package provides, because the library crate’s code can be shared.
The module tree should be defined in src/lib.rs. Then, any public items can be used in the binary crate by starting paths with the name of the package. The binary crate becomes a user of the library crate just like a completely external crate would use the library crate: it can only use the public API. This helps you design a good API; not only are you the author, you’re also a client!
In Chapter 12, we’ll demonstrate this organizational practice with a command-line program that will contain both a binary crate and a library crate.
Starting Relative Paths with super
We can construct relative paths that begin in the parent module, rather than
the current module or the crate root, by using super
at the start of the
path. This is like starting a filesystem path with the ..
syntax. Using
super
allows us to reference an item that we know is in the parent module,
which can make rearranging the module tree easier when the module is closely
related to the parent, but the parent might be moved elsewhere in the module
tree someday.
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
defined in the back_of_house
module calls the
function deliver_order
defined in the parent module by specifying the path to
deliver_order
starting with super
:
Filename: src/lib.rs
fn deliver_order() {}
mod back_of_house {
fn fix_incorrect_order() {
cook_order();
super::deliver_order();
}
fn cook_order() {}
}
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 deliver_order
and find it.
Success! We think the back_of_house
module and the deliver_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 details extra to the usage of pub
with structs and enums. 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");
}
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;
}
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
.