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[rustc.git] / src / doc / book / src / ch07-03-paths-for-referring-to-an-item-in-the-module-tree.md

## 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 (for code
  from an external crate) or a literal `crate` (for code from the current
  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? 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. 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”][pub]<!-- ignore --> 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.

<span class="filename">Filename: src/lib.rs</span>

```rust,ignore,does_not_compile
{{#rustdoc_include ../listings/ch07-managing-growing-projects/listing-07-03/src/lib.rs}}
```

<span class="caption">Listing 7-3: Calling the `add_to_waitlist` function using
absolute and relative paths</span>

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 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.

```console
{{#include ../listings/ch07-managing-growing-projects/listing-07-03/output.txt}}
```

<span class="caption">Listing 7-4: Compiler errors from building the code in
Listing 7-3</span>

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.

<span class="filename">Filename: src/lib.rs</span>

```rust,ignore,does_not_compile
{{#rustdoc_include ../listings/ch07-managing-growing-projects/listing-07-05/src/lib.rs}}
```

<span class="caption">Listing 7-5: Declaring the `hosting` module as `pub` to
use it from `eat_at_restaurant`</span>

Unfortunately, the code in Listing 7-5 still results in an error, as shown in
Listing 7-6.

```console
{{#include ../listings/ch07-managing-growing-projects/listing-07-05/output.txt}}
```

<span class="caption">Listing 7-6: Compiler errors from building the code in
Listing 7-5</span>

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.

<span class="filename">Filename: src/lib.rs</span>

```rust,noplayground,test_harness
{{#rustdoc_include ../listings/ch07-managing-growing-projects/listing-07-07/src/lib.rs}}
```

<span class="caption">Listing 7-7: Adding the `pub` keyword to `mod hosting`
and `fn add_to_waitlist` lets us call the function from
`eat_at_restaurant`</span>

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!

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 about how they
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][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 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][ch12]<!-- ignore -->, 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 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` 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`:

<span class="filename">Filename: src/lib.rs</span>

```rust,noplayground,test_harness
{{#rustdoc_include ../listings/ch07-managing-growing-projects/listing-07-08/src/lib.rs}}
```

<span class="caption">Listing 7-8: Calling a function using a relative path
starting with `super`</span>

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 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.

<span class="filename">Filename: src/lib.rs</span>

```rust,noplayground
{{#rustdoc_include ../listings/ch07-managing-growing-projects/listing-07-09/src/lib.rs}}
```

<span class="caption">Listing 7-9: A struct with some public fields and some
private fields</span>

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.

<span class="filename">Filename: src/lib.rs</span>

```rust,noplayground
{{#rustdoc_include ../listings/ch07-managing-growing-projects/listing-07-10/src/lib.rs}}
```

<span class="caption">Listing 7-10: Designating an enum as public makes all its
variants public</span>

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`.

[pub]: ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html#exposing-paths-with-the-pub-keyword
[api-guidelines]: https://rust-lang.github.io/api-guidelines/
[ch12]: ch12-00-an-io-project.html