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.
{{#rustdoc_include ../listings/ch07-managing-growing-projects/listing-07-03/src/lib.rs}}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 it 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 moved 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 errors we get are shown in Listing 7-4.
{{#include ../listings/ch07-managing-growing-projects/listing-07-03/output.txt}}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.
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.
{{#rustdoc_include ../listings/ch07-managing-growing-projects/listing-07-05/src/lib.rs}}Unfortunately, the code in Listing 7-5 still results in compiler errors, as shown in Listing 7-6.
{{#include ../listings/ch07-managing-growing-projects/listing-07-05/output.txt}}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.
{{#rustdoc_include ../listings/ch07-managing-growing-projects/listing-07-07/src/lib.rs}}Now the code will compile! To see why adding the pub keyword lets us use
these paths in eat_at_restaurant 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.
We mentioned that 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 within the library crate. This lets other projects benefit from most of the 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.
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.
{{#rustdoc_include ../listings/ch07-managing-growing-projects/listing-07-08/src/lib.rs}}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.
We can also use pub to designate structs and enums as public, but there are a
few extra details 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.
{{#rustdoc_include ../listings/ch07-managing-growing-projects/listing-07-09/src/lib.rs}}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.
{{#rustdoc_include ../listings/ch07-managing-growing-projects/listing-07-10/src/lib.rs}}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.