The Rust Programming Language

The state pattern is an object-oriented design pattern. The crux of the pattern is that a value has some internal state, represented by a set of state objects, and the value’s behavior changes based on the internal state. The state objects share functionality--in Rust, of course, we use structs and traits rather than objects and inheritance. Each state object representing the state is responsible for its own behavior and for governing when it should change into another state. The value that holds a state object knows nothing about the different behavior of the states or when to transition between states.

Using the state pattern means when the business requirements of the program change, we won’t need to change the code of the value holding the state or the code that uses the value. We’ll only need to update the code inside one of the state objects to change its rules, or perhaps add more state objects. Let’s look at an example of the state design pattern and how to use it in Rust.

To explore this idea, we’ll implement a blog post workflow in an incremental way. The blog’s final functionality will look like this:

1. A blog post starts as an empty draft.
2. Once the draft is done, a review of the post is requested.
3. Once the post is approved, it gets published.
4. Only published blog posts return content to print, so unapproved posts can’t accidentally be published.

Any other changes attempted on a post should have no effect. For example, if we try to approve a draft blog post before we’ve requested a review, the post should stay an unpublished draft.

Listing 17-11 shows this workflow in code form. This is an example usage of the API we’re going to implement in a library crate named blog:

Filename: src/main.rs

extern crate blog;
use blog::Post;

fn main() {
    let mut post = Post::new();

    post.add_text("I ate a salad for lunch today");
    assert_eq!("", post.content());

    post.request_review();
    assert_eq!("", post.content());

    post.approve();
    assert_eq!("I ate a salad for lunch today", post.content());
}

Listing 17-11: Code that demonstrates the desired behavior we want our blog crate to have

We want to allow the user to create a new draft blog post with Post::new. Then, we want to allow text to be added to the blog post while it’s in the draft state. If we try to get the post’s content immediately, before approval, nothing should happen because the post is still a draft. We’ve added an assert_eq! here for demonstration purposes. An excellent unit test for this would be to assert that a draft blog post returns an empty string from the content method, but we’re not going to write tests for this example.

Next, we want to enable a request for a review of the post, and we want content to return an empty string while waiting for the review. Lastly, when the post receives approval, it should get published, meaning the text of the post will be returned when content is called.

Notice that the only type we’re interacting with from the crate is the Post type. This type will use the state pattern and will hold a value that will be one of three state objects representing the various states a post can be in---draft, waiting for review, or published. Changing from one state to another will be managed internally within the Post type. The states change in response to the methods users of our library call on the Post instance, but they don’t have to manage the state changes directly. This also means users can’t make a mistake with the states, like publishing a post before it is reviewed.

Let’s get started on the implementation of the library! We know we need a public Post struct that holds some content, so let’s start with the definition of the struct and an associated public new function to create an instance of Post, as shown in Listing 17-12. We’ll also make a private State trait. Then Post will hold a trait object of Box<State> inside an Option in a private field named state. We’ll see why the Option is necessary in a bit.

The State trait defines the behavior shared by different post states, and the Draft, PendingReview, and Published states will all implement the State trait. For now, the trait does not have any methods, and we’re going to start by defining just the Draft state since that’s the state we want a post to start in:

Filename: src/lib.rs

# #![allow(unused_variables)]
#fn main() {
pub struct Post {
    state: Option<Box<State>>,
    content: String,
}

impl Post {
    pub fn new() -> Post {
        Post {
            state: Some(Box::new(Draft {})),
            content: String::new(),
        }
    }
}

trait State {}

struct Draft {}

impl State for Draft {}
#}

Listing 17-12: Definition of a Post struct and a new function that creates a new Post instance, a State trait, and a Draft struct

When we create a new Post, we set its state field to a Some value that holds a Box. This Box points to a new instance of the Draft struct. This ensures whenever we create a new instance of Post, it’ll start out as a draft. Because the state field of Post is private, there’s no way to create a Post in any other state!

In the Post::new function, we set the content field to a new, empty String. Listing 17-11 showed that we want to be able to call a method named add_text and pass it a &str that’s then added to the text content of the blog post. We implement this as a method rather than exposing the content field as pub. This means we can implement a method later that will control how the content field’s data is read. The add_text method is pretty straightforward, so let’s add the implementation in Listing 17-13 to the impl Post block:

Filename: src/lib.rs

# #![allow(unused_variables)]
#fn main() {
# pub struct Post {
#     content: String,
# }
#
impl Post {
    // --snip--
    pub fn add_text(&mut self, text: &str) {
        self.content.push_str(text);
    }
}
#}

Listing 17-13: Implementing the add_text method to add text to a post’s content

add_text takes a mutable reference to self, since we’re changing the Post instance that we’re calling add_text on. We then call push_str on the String in content and pass the text argument to add to the saved content. This behavior doesn’t depend on the state the post is in so it’s not part of the state pattern. The add_text method doesn’t interact with the state field at all, but it is part of the behavior we want to support.

Even after we’ve called add_text and added some content to our post, we still want the content method to return an empty string slice since the post is still in the draft state, as shown on line 8 of Listing 17-11. For now, let’s implement the content method with the simplest thing that will fulfill this requirement: always returning an empty string slice. We’re going to change this later once we implement the ability to change a post’s state so it can be published. So far, though, posts can only be in the draft state, so the post content should always be empty. Listing 17-14 shows this placeholder implementation:

Filename: src/lib.rs

# #![allow(unused_variables)]
#fn main() {
# pub struct Post {
#     content: String,
# }
#
impl Post {
    // --snip--
    pub fn content(&self) -> &str {
        ""
    }
}
#}

Listing 17-14: Adding a placeholder implementation for the content method on Post that always returns an empty string slice

With this added content method, everything in Listing 17-11 up to line 8 works as we intend.

Next up we need to add functionality to request a review of a post, which should change its state from Draft to PendingReview. We want to give Post a public method named request_review that will take a mutable reference to self. Then we’re going to call an internal request_review method on the current state of Post, and this second request_review method will consume the current state and return a new state. Listing 17-15 shows this code:

Filename: src/lib.rs

# #![allow(unused_variables)]
#fn main() {
# pub struct Post {
#     state: Option<Box<State>>,
#     content: String,
# }
#
impl Post {
    // --snip--
    pub fn request_review(&mut self) {
        if let Some(s) = self.state.take() {
            self.state = Some(s.request_review())
        }
    }
}

trait State {
    fn request_review(self: Box<Self>) -> Box<State>;
}

struct Draft {}

impl State for Draft {
    fn request_review(self: Box<Self>) -> Box<State> {
        Box::new(PendingReview {})
    }
}

struct PendingReview {}

impl State for PendingReview {
    fn request_review(self: Box<Self>) -> Box<State> {
        self
    }
}
#}

Listing 17-15: Implementing request_review methods on Post and the State trait

We’ve added the request_review method to the State trait; all types that implement the trait will now need to implement the request_review method. Note that rather than having self, &self, or &mut self as the first parameter of the method, we have self: Box<Self>. This syntax means the method is only valid when called on a Box holding the type. This syntax takes ownership of Box<Self>, invalidating the old state so that the state value of the Post can transform itself into a new state.

To consume the old state, the request_review method needs to take ownership of the state value. This is where the Option in the state field of Post comes in: we call the take method to take the Some value out of the state field and leave a None in its place, since Rust doesn’t let us have unpopulated fields in structs. This lets us move the state value out of Post rather than borrowing it. Then we’ll set the post’s state value to the result of this operation.

We need to set state to None temporarily, rather than code like self.state = self.state.request_review(); that would set the state field directly, to get ownership of the state value. This ensures Post can’t use the old state value after we’ve transformed it into a new state.

The request_review method on Draft needs to return a new, boxed instance of a new PendingReview struct, which represents the state when a post is waiting for a review. The PendingReview struct also implements the request_review method, but doesn’t do any transformations. Rather, it returns itself, since when we request a review on a post already in the PendingReview state, it should stay in the PendingReview state.

Now we can start seeing the advantages of the state pattern: the request_review method on Post is the same no matter its state value. Each state is responsible for its own rules.

We’re going to leave the content method on Post as it is, returning an empty string slice. We can now have a Post in the PendingReview state as well as the Draft state, but we want the same behavior in the PendingReview state. Listing 17-11 now works up until line 11!

The approve method will be similar to the request_review method: it will set state to the value that the current state says it should have when that state is approved, shown in Listing 17-16.

Filename: src/lib.rs

# #![allow(unused_variables)]
#fn main() {
# pub struct Post {
#     state: Option<Box<State>>,
#     content: String,
# }
#
impl Post {
    // --snip--
    pub fn approve(&mut self) {
        if let Some(s) = self.state.take() {
            self.state = Some(s.approve())
        }
    }
}

trait State {
    fn request_review(self: Box<Self>) -> Box<State>;
    fn approve(self: Box<Self>) -> Box<State>;
}

struct Draft {}

impl State for Draft {
#     fn request_review(self: Box<Self>) -> Box<State> {
#         Box::new(PendingReview {})
#     }
#
    // --snip--
    fn approve(self: Box<Self>) -> Box<State> {
        self
    }
}

struct PendingReview {}

impl State for PendingReview {
#     fn request_review(self: Box<Self>) -> Box<State> {
#         self
#     }
#
    // --snip--
    fn approve(self: Box<Self>) -> Box<State> {
        Box::new(Published {})
    }
}

struct Published {}

impl State for Published {
    fn request_review(self: Box<Self>) -> Box<State> {
        self
    }

    fn approve(self: Box<Self>) -> Box<State> {
        self
    }
}
#}

Listing 17-16: Implementing the approve method on Post and the State trait

We add the approve method to the State trait, and add a new struct that implements State, the Published state.

Similar to request_review, if we call the approve method on a Draft, it will have no effect since it will return self. When we call approve on PendingReview, it returns a new, boxed instance of the Published struct. The Published struct implements the State trait, and for both the request_review method and the approve method, it returns itself, since the post should stay in the Published state in those cases.

Now to update the content method on Post: if the state is Published we want to return the value in the post’s content field; otherwise we want to return an empty string slice, as shown in Listing 17-17:

Filename: src/lib.rs

# #![allow(unused_variables)]
#fn main() {
# trait State {
#     fn content<'a>(&self, post: &'a Post) -> &'a str;
# }
# pub struct Post {
#     state: Option<Box<State>>,
#     content: String,
# }
#
impl Post {
    // --snip--
    pub fn content(&self) -> &str {
        self.state.as_ref().unwrap().content(&self)
    }
    // --snip--
}
#}

Listing 17-17: Updating the content method on Post to delegate to a content method on State

Because the goal is to keep all these rules inside the structs that implement State, we call a content method on the value in state and pass the post instance (that is, self) as an argument. Then we return the value that’s returned from using the content method on the state value.

We call the as_ref method on the Option because we want a reference to the value inside the Option rather than ownership of it. Because state is an Option<Box<State>>, calling as_ref returns an Option<&Box<State>>. If we didn’t call as_ref, we’d get an error because we can’t move state out of the borrowed &self of the function parameter.

We’re then calling the unwrap method, which we know will never panic, because we know the methods on Post ensure that state will always contain a Some value when those methods are done. This is one of the cases we talked about in Chapter 12 when we know that a None value is never possible, even though the compiler isn’t able to understand that.

So then we have a &Box<State>, and when we call the content on it, deref coercion will take effect on the & and the Box so that the content method will ultimately be called on the type that implements the State trait.

That means we need to add content to the State trait definition, and that’s where we’ll put the logic for what content to return depending on which state we have, as shown in Listing 17-18:

Filename: src/lib.rs

# #![allow(unused_variables)]
#fn main() {
# pub struct Post {
#     content: String
# }
trait State {
    // --snip--
    fn content<'a>(&self, post: &'a Post) -> &'a str {
        ""
    }
}

// --snip--
struct Published {}

impl State for Published {
    // --snip--
    fn content<'a>(&self, post: &'a Post) -> &'a str {
        &post.content
    }
}
#}

Listing 17-18: Adding the content method to the State trait

We add a default implementation for the content method that returns an empty string slice. That means we don’t need to implement content on the Draft and PendingReview structs. The Published struct will override the content method and return the value in post.content.

Note that we need lifetime annotations on this method, like we discussed in Chapter 10. We’re taking a reference to a post as an argument, and returning a reference to part of that post, so the lifetime of the returned reference is related to the lifetime of the post argument.

And we’re done-- all of Listing 17-11 now works! We’ve implemented the state pattern with the rules of the blog post workflow. The logic around the rules lives in the state objects rather than scattered throughout Post.

We’ve shown that Rust is capable of implementing the object-oriented state pattern to encapsulate the different kinds of behavior a post should have in each state. The methods on Post know nothing about the different kinds of behavior. The way this code is organized, we only have to look in one place to know the different ways a published post can behave: the implementation of the State trait on the Published struct.

If we were to create an alternative implementation that didn’t use the state pattern we might use match statements in the methods on Post, or even in the main code that checks the state of the post and changes behavior in those places instead. That would mean we’d have to look in a lot of places to understand all the implications of a post being in the published state! This would only increase the more states we added: each of those match statements would need another arm.

With the state pattern, the Post methods and the places we use Post don’t need match statements, and to add a new state we would only need to add a new struct and implement the trait methods on that one struct.

This implementation is easy to extend to add more functionality. To see the simplicity of maintaining code that uses this patterns, try out a few of these suggestions:

• Allow users to add text content only when a post is in the Draft state
• Add a reject method that changes the post’s state from PendingReview back to Draft
• Require two calls to approve before the state can be changed to Published

One downside of the state pattern is that, because the states implement the transitions between states, some of the states are coupled to each other. If we add another state between PendingReview and Published, such as Scheduled, we would have to change the code in PendingReview to transition to Scheduled instead. It would be less work if PendingReview wouldn’t need to change with the addition of a new state, but that would mean switching to another design pattern.

Another downside is that we find ourselves with a few bits of duplicated logic. To eliminate this, we might try to make default implementations for the request_review and approve methods on the State trait that return self, but this would violate object safety, since the trait doesn’t know what the concrete self will be exactly. We want to be able to use State as a trait object, so we need its methods to be object safe.

The other duplication is the similar implementations of the request_review and approve methods on Post. Both methods delegate to the implementation of the same method on the value in the state field of Option, and set the new value of the state field to the result. If we had a lot of methods on Post that followed this pattern, we might consider defining a macro to eliminate the repetition (see Appendix D on macros).

By implementing this pattern exactly as it’s defined for object-oriented languages, we’re not taking full advantage of Rust’s strengths as much as we could. Let’s take a look at some changes we can make to this code that can make invalid states and transitions into compile time errors.

We’re going to show how to rethink the state pattern to get a different set of tradeoffs. Rather than encapsulating the states and transitions completely so that outside code has no knowledge of them, we’re going to encode the states into different types. Like this, Rust’s type checking system will make attempts to use draft posts where only published posts are allowed into a compiler error.

Let’s consider the first part of main from Listing 17-11:

Filename: src/main.rs

fn main() {
    let mut post = Post::new();

    post.add_text("I ate a salad for lunch today");
    assert_eq!("", post.content());
}

We still enable the creation of new posts in the draft state using Post::new, and the ability to add text to the post’s content. But instead of having a content method on a draft post that returns an empty string, we’ll make it so that draft posts don’t have the content method at all. That way, if we try to get a draft post’s content, we’ll get a compiler error telling us the method doesn’t exist. This will make it impossible for us to accidentally display draft post content in production, since that code won’t even compile. Listing 17-19 shows the definition of a Post struct, a DraftPost struct, and methods on each:

Filename: src/lib.rs

# #![allow(unused_variables)]
#fn main() {
pub struct Post {
    content: String,
}

pub struct DraftPost {
    content: String,
}

impl Post {
    pub fn new() -> DraftPost {
        DraftPost {
            content: String::new(),
        }
    }

    pub fn content(&self) -> &str {
       &self.content
    }
}

impl DraftPost {
    pub fn add_text(&mut self, text: &str) {
        self.content.push_str(text);
    }
}
#}

Listing 17-19: A Post with a content method and a DraftPost without a content method

Both the Post and DraftPost structs have a private content field that stores the blog post text. The structs no longer have the state field since we’re moving the encoding of the state to the types of the structs. Post will represent a published post, and it has a content method that returns the content.

We still have a Post::new function, but instead of returning an instance of Post, it returns an instance of DraftPost. Because content is private, and there aren’t any functions that return Post, it’s not possible to create an instance of Post right now.

DraftPost has an add_text method so we can add text to content as before, but note that DraftPost does not have a content method defined! So now the program ensures all posts start as draft posts, and draft posts don’t have their content available for display. Any attempt to get around these constraints will result in a compiler error.

So how do we get a published post then? We want to enforce the rule that a draft post has to be reviewed and approved before it can be published. A post in the pending review state should still not display any content. Let’s implement these constraints by adding another struct, PendingReviewPost, defining the request_review method on DraftPost to return a PendingReviewPost, and defining an approve method on PendingReviewPost to return a Post as shown in Listing 17-20:

Filename: src/lib.rs

# #![allow(unused_variables)]
#fn main() {
# pub struct Post {
#     content: String,
# }
#
# pub struct DraftPost {
#     content: String,
# }
#
impl DraftPost {
    // --snip--

    pub fn request_review(self) -> PendingReviewPost {
        PendingReviewPost {
            content: self.content,
        }
    }
}

pub struct PendingReviewPost {
    content: String,
}

impl PendingReviewPost {
    pub fn approve(self) -> Post {
        Post {
            content: self.content,
        }
    }
}
#}

Listing 17-20: A PendingReviewPost that gets created by calling request_review on DraftPost, and an approve method that turns a PendingReviewPost into a published Post

The request_review and approve methods take ownership of self, thus consuming the DraftPost and PendingReviewPost instances and transforming them into a PendingReviewPost and a published Post, respectively. This way, we won’t have any DraftPost instances lingering around after we’ve called request_review on them, and so forth. PendingReviewPost doesn’t have a content method defined on it, so attempting to read its content results in a compiler error, as with DraftPost. Because the only way to get a published Post instance that does have a content method defined is to call the approve method on a PendingReviewPost, and the only way to get a PendingReviewPost is to call the request_review method on a DraftPost, we’ve now encoded the blog post workflow into the type system.

This does mean we have to make some small changes to main. The request_review and approve methods return new instances rather than modifying the struct they’re called on, so we need to add more let post = shadowing assignments to save the returned instances. We also can’t have the assertions about the draft and pending review post’s contents being empty strings, nor do we need them: we can’t compile code that tries to use the content of posts in those states any longer. The updated code in main is shown in Listing 17-21:

Filename: src/main.rs

extern crate blog;
use blog::Post;

fn main() {
    let mut post = Post::new();

    post.add_text("I ate a salad for lunch today");

    let post = post.request_review();

    let post = post.approve();

    assert_eq!("I ate a salad for lunch today", post.content());
}

Listing 17-21: Modifications to main to use the new implementation of the blog post workflow

These changes we need to make to main to reassign post means this implementation doesn’t quite follow the object-oriented state pattern anymore: the transformations between the states are no longer encapsulated entirely within the Post implementation. However, our gain is that invalid states are now impossible because of the type system and the type checking that happens at compile time! This ensures that certain bugs, such as the content of an unpublished post being displayed, will be discovered before they make it to production.

Try the tasks suggested for additional requirements that we mentioned at the start of this section on this code, to see how working with this version of the code feels.

We’ve seen that even though Rust is capable of implementing object-oriented design patterns, other patterns like encoding state into the type system are also available in Rust. These patterns have different tradeoffs. While you may be very familiar with object-oriented patterns, rethinking the problem in order to take advantage of Rust’s features can provide benefits like preventing some bugs at compile-time. Object-oriented patterns won’t always be the best solution in Rust, because of the features like ownership that object-oriented languages don’t have.

No matter whether you think Rust is an object-oriented language or not after reading this chapter, you’ve now seen that trait objects are a way to get some object-oriented features in Rust. Dynamic dispatch can give your code some flexibility in exchange for a bit of runtime performance. This flexibility can be used to implement object-oriented patterns that can help with the maintainability of your code. Rust also has other different features, like ownership, that object-oriented languages don’t have. An object-oriented pattern won’t always be the best way to take advantage of Rust’s strengths, but is an available option.

Next, let’s look at another feature of Rust that enables lots of flexibility: patterns. We’ve looked at them briefly throughout the book, but haven’t seen everything they’re capable of yet. Let’s go!