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What is the typestate pattern? #

The Typestate Pattern in Rust is a way to manage objects that go through different states in their lifecycle. It leverages Rust’s powerful type system to enforce these states and transitions between them, making your code safer and more predictable.

Here are the key ideas behind the Typestate Pattern:

  • States as structs: Each possible state of the object is represented by a separate struct. This lets you associate specific methods and data with each state.
  • Transitions with ownership: Methods that transition the object to a new state consume the old state and return a value representing the new state. Rust’s ownership system ensures you can’t accidentally use the object in an invalid state.
  • Encapsulated functionality: Methods are only available on the structs representing the valid states. This prevents you from trying to perform actions that aren’t allowed in the current state.

Benefits of using the Typestate Pattern:

  • Safer code: By statically checking types at compile time, the compiler prevents you from accidentally using the object in an invalid state. This leads to fewer runtime errors and more robust code.
  • Improved readability: The code becomes more self-documenting because the valid state transitions are encoded in the types themselves.
  • Clearer APIs: By separating functionality based on state, APIs become more intuitive and easier to understand.

More resources on typestate pattern and others in Rust #

YouTube video for this article #

This blog post has short examples on how to use the typestate pattern in Rust. If you like to learn via video, please watch the companion video on the developerlife.com YouTube channel.


Examples of typestate pattern in Rust #

Let’s create some examples to illustrate how to use the typestate pattern in Rust. You can run cargo new --bin typestate-pattern to create a new binary crate.

The code in the video and this tutorial are all in this GitHub repo.

Then add the following to the Cargo.toml file that’s generated. These pull in all the dependencies that we need for these examples.

[package]
name = "typestate-pattern"
version = "0.1.0"
edition = "2021"

[[bin]]
name = "ex1"
path = "src/ex1.rs"

[[bin]]
name = "ex2"
path = "src/ex2.rs"

[[bin]]
name = "ex3"
path = "src/ex3.rs"

[[bin]]
name = "ex3_1"
path = "src/ex3_1.rs"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
crossterm = { version = "0.27.0", features = ["event-stream"] }

Example 1: Simple version of this is using enums to encapsulate states as variants #

Then you can add the following code to the src/ex1.rs file.

#[derive(Debug)]
pub enum InputEvent {
    Keyboard((KeyPress, Option<Vec<Modifier>>)),
    Resize(Size),
    Mouse(MouseEvent),
}

#[derive(Debug)]
pub enum Modifier {
    Shift,
    Control,
    Alt,
}

#[derive(Debug)]
pub enum KeyPress {
    Char(char),
    Enter,
    Backspace,
    Delete,
    Left,
    Right,
    Up,
    Down,
    Home,
    End,
    PageUp,
    PageDown,
    Tab,
    F(u8),
}

#[derive(Debug)]
pub enum Size {
    Height(u16),
    Width(u16),
}

#[derive(Debug)]
pub enum MouseEvent {
    Press(MouseButton, u16, u16),
    Release(u16, u16),
    Hold(u16, u16),
}

#[derive(Debug)]
pub enum MouseButton {
    Left,
    Right,
    Middle,
}

impl InputEvent {
    pub fn pretty_print(&self) {
        let it = match self {
            InputEvent::Keyboard((keypress, modifiers)) => {
                let mut result = format!("{:?}", keypress);
                if let Some(modifiers) = modifiers {
                    result.push_str(&format!("{:?}", modifiers));
                }
                result
            }
            InputEvent::Resize(size) => format!("{:?}", size),
            InputEvent::Mouse(mouse_event) => format!("{:?}", mouse_event),
        };
        println!("{}", it);
    }
}

fn main() {
    let a_pressed = InputEvent::Keyboard((KeyPress::Char('a'), None));
    println!("{:?}", a_pressed);

    let ctrl_c_pressed = InputEvent::Keyboard(
        (KeyPress::Char('c'), Some(vec![Modifier::Control]))
    );
    println!("{:?}", ctrl_c_pressed);

    let enter_pressed = InputEvent::Keyboard((KeyPress::Enter, None));
    enter_pressed.pretty_print();

    let mouse_pressed = InputEvent::Mouse(
        MouseEvent::Press(MouseButton::Left, 10, 20)
    );
    mouse_pressed.pretty_print();
}

The code for this example is here. Here’s the code for the real InputEvent.

The main things to note about this code.

  • We have a bunch of enums that represent different types of input events.
  • We have a method on the InputEvent enum that pretty prints the event for all variants. We don’t have a way to restrict methods on a specific variant using this approach.

When you run this code (using cargo run --bin ex1), it should produce the following output:

$ cargo run --bin ex1
Keyboard((Char('a'), None))
Keyboard((Char('c'), Some([Control])))
Enter
Press(Left, 10, 20)

Example 2: Slightly more complex versions are where one type + data = another type #

For this example, let’s add the following code to the src/ex2.rs file.

mod ex1;
use ex1::InputEvent;

#[derive(Debug)]
pub enum EditorEvent {
    InsertChar(char),
    InsertNewLine,
    Delete,
    Backspace,
    MoveCursorLeft,
    MoveCursorRight,
    MoveCursorUp,
    MoveCursorDown,
    Copy,
    Paste,
    Cut,
    Undo,
    Redo,
}

impl TryFrom<InputEvent> for EditorEvent {
    type Error = String;

    fn try_from(input_event: InputEvent) -> Result<Self, Self::Error> {
        match input_event {
            InputEvent::Keyboard((keypress, modifiers)) =>
                match (keypress, modifiers)
            {
                (ex1::KeyPress::Char(ch), None) => Ok(Self::InsertChar(ch)),
                (ex1::KeyPress::Char(_), Some(_)) => todo!(),
                (ex1::KeyPress::Enter, None) => Ok(Self::InsertNewLine),
                (ex1::KeyPress::Enter, Some(_)) => todo!(),
                (ex1::KeyPress::Backspace, None) => todo!(),
                (ex1::KeyPress::Backspace, Some(_)) => todo!(),
                (ex1::KeyPress::Delete, None) => todo!(),
                (ex1::KeyPress::Delete, Some(_)) => todo!(),
                (ex1::KeyPress::Left, None) => todo!(),
                (ex1::KeyPress::Left, Some(_)) => todo!(),
                (ex1::KeyPress::Right, None) => todo!(),
                (ex1::KeyPress::Right, Some(_)) => todo!(),
                (ex1::KeyPress::Up, None) => todo!(),
                (ex1::KeyPress::Up, Some(_)) => todo!(),
                (ex1::KeyPress::Down, None) => todo!(),
                (ex1::KeyPress::Down, Some(_)) => todo!(),
                (ex1::KeyPress::Home, None) => todo!(),
                (ex1::KeyPress::Home, Some(_)) => todo!(),
                (ex1::KeyPress::End, None) => todo!(),
                (ex1::KeyPress::End, Some(_)) => todo!(),
                (ex1::KeyPress::PageUp, None) => todo!(),
                (ex1::KeyPress::PageUp, Some(_)) => todo!(),
                (ex1::KeyPress::PageDown, None) => todo!(),
                (ex1::KeyPress::PageDown, Some(_)) => todo!(),
                (ex1::KeyPress::Tab, None) => todo!(),
                (ex1::KeyPress::Tab, Some(_)) => todo!(),
                (ex1::KeyPress::F(_), None) => todo!(),
                (ex1::KeyPress::F(_), Some(_)) => todo!(),
            },
            InputEvent::Resize(_) => todo!(),
            InputEvent::Mouse(_) => todo!(),
        }
    }
}

fn main() {
    let a_pressed = InputEvent::Keyboard((ex1::KeyPress::Char('a'), None));
    println!("{:?}", EditorEvent::try_from(a_pressed));

    let enter_pressed = InputEvent::Keyboard((ex1::KeyPress::Enter, None));
    println!("{:?}", EditorEvent::try_from(enter_pressed));
}

You can get the source code for this example here. Here’s the code for the real EditorEvent.

Here are some notes on this code:

  • We have a new enum called EditorEvent that represents different types of events that can happen in an editor.
  • We have a TryFrom implementation for InputEvent that converts an InputEvent into an EditorEvent. This is a way to restrict methods to specific variants of an enum by converting it into a totally different type.
  • We still don’t have a way to restrict methods to specific variants of the enum.

When you run this code (using cargo run --bin ex2), it should produce the following:

$ cargo run --bin ex2
Ok(InsertChar('a'))
Ok(InsertNewLine)

Example 3: Best of both worlds, using generics and struct / enum with a marker trait #

Finally we have arrived at the typestate pattern in Rust. With this example:

  • You can now group all the states under a marker.
  • You can have methods that are specific to a variant.
  • You can specify methods that are common to all.
  • It’s like a very sophisticated builder pattern if you’re already familiar with that.

Add the following code to the src/ex3.rs file.

use self::type_state_builder::HttpResponse;
use crossterm::style::Stylize;

pub fn main() -> Result<(), String> {
    let response = HttpResponse::<()>::new();
    println!("{}", "Start state".red().bold().underlined());
    println!("response: {:#?}", response);
    println!(
        "response size: {}",
        response.get_size().to_string().blue().bold()
    );

    // Transition to HeaderAndBody state by calling `set_status_line`.
    let mut response = response.set_status_line(200, "OK");
    println!("response: {:#?}", response);

    // Status line is required.
    println!("{}", "HeaderAndBody state".red().bold().underlined());
    println!("response_code: {}", response.get_response_code());
    println!("response body: {:#?}", response.get_body());
    println!("response: {:#?}", response);
    println!(
        "response size: {}",
        response.get_size().to_string().blue().bold()
    );

    // Body and headers are optional.
    println!("{}", "HeaderAndBody state # 2".red().bold().underlined());
    response.add_header("Content-Type", "text/html");
    response.set_body("<html><body>Hello World!</body></html>");
    println!("response: {:#?}", response);
    println!(
        "response size: {}",
        response.get_size().to_string().blue().bold()
    );

    // Final state.
    println!("{}", "Final state".red().bold().underlined());
    let response = response.finish();
    println!("response_code: {}", response.get_response_code());
    println!("status_line: {}", response.get_status_line());
    println!("headers: {:#?}", response.get_headers());
    println!("body: {}", response.get_body());
    println!("response: {:#?}", response);
    println!(
        "response size: {}",
        response.get_size().to_string().blue().bold()
    );

    Ok(())
}

Note the API that we have built here:

  • You can’t call get_response_code or get_body until you’ve called set_status_line.
  • You can’t call add_header or set_body until you’ve called set_status_line.
  • You can’t call finish until you’ve called set_status_line.
  • We have 3 states: Start, HeaderAndBody, and Final. These are meant to be used as markers to restrict methods to specific states. Each is a struct with a marker trait. And it may or may not contain data / fields.
  • We have a HttpResponse struct that uses a generic type T: Marker to represent the state. This is a way to restrict methods to specific states.
  • We can transition between states by calling methods that consume the current state and return a new state. These methods are specific to the state they transition from. And they can be implemented via impl HttpResponse<T: Marker> { ... } blocks, where T is the Start, HeaderAndBody, or Final state.
  • We can even implement methods that are valid for a non-existent state using impl HttpResponse<()> { ... }. This is the constructor.
  • In the Final state, the data becomes immutable.

Add the following code to desribe the different state structs.

pub mod state {
    #[derive(Debug, Clone, Default)]
    pub struct Start {}

    #[derive(Debug, Clone, Default)]
    pub struct HeaderAndBody {
        pub response_code: u8,
        pub status_line: String,
        pub headers: Option<Vec<(String, String)>>,
        pub body: Option<String>,
    }

    #[derive(Debug, Clone, Default)]
    pub struct Final {
        pub response_code: u8,
        pub status_line: String,
        pub headers: Vec<(String, String)>,
        pub body: String,
    }

    // The following marker trait is used to restrict the operations
    // that are available in each state. This isn't strictly necessary,
    // but it's a nice thing to use in a where clause to restrict types.
    pub trait Marker {}
    impl Marker for () {}
    impl Marker for Start {}
    impl Marker for HeaderAndBody {}
    impl Marker for Final {}
}

Here is the code for the HttpResponse struct.

pub mod type_state_builder {
    use super::state::{Final, HeaderAndBody, Marker, Start};

    #[derive(Debug, Clone, Default)]
    pub struct HttpResponse<S: Marker> {
        pub state: S,
    }

    // Operations that are available in all states.
    impl<S> HttpResponse<S>
    where
        S: Marker,
    {
        pub fn get_size(&self) -> String {
            let len = std::mem::size_of_val(self);
            format!("{} bytes", len)
        }
    }

    // Operations that are only valid in `()`.
    impl HttpResponse<()> {
        pub fn new() -> HttpResponse<Start> {
            HttpResponse { state: Start {} }
        }
    }

    // Operations that are only valid in `Start`.
    impl HttpResponse<Start> {
        pub fn set_status_line(
            self,
            response_code: u8,
            message: &str,
        ) -> HttpResponse<HeaderAndBody> {
            HttpResponse {
                state: HeaderAndBody {
                    response_code,
                    status_line: format!(
                        "HTTP/1.1 {} {}", response_code, message
                    ),
                    ..Default::default()
                },
            }
        }
    }

    // Operations that are only valid in `HeaderAndBodyState`.
    impl HttpResponse<HeaderAndBody> {
        // setter.
        pub fn add_header(&mut self, key: &str, value: &str) {
            if self.state.headers.is_none() {
                self.state.headers.replace(Vec::new());
            }
            if let Some(v) = self.state.headers.as_mut() {
                v.push((key.to_string(), value.to_string()))
            }
        }

        // getter.
        pub fn get_response_code(&self) -> u8 {
            self.state.response_code
        }

        // setter.
        pub fn set_body(&mut self, body: &str) {
            self.state.body.replace(body.to_string());
        }

        // getter.
        pub fn get_body(&self) -> Option<&str> {
            self.state.body.as_deref()
        }

        // transition to Final state.
        pub fn finish(mut self) -> HttpResponse<Final> {
            HttpResponse {
                state: Final {
                    response_code: self.state.response_code,
                    status_line: self.state.status_line.clone(),
                    headers: self.state.headers.take().unwrap_or_default(),
                    body: self.state.body.take().unwrap_or_default(),
                },
            }
        }
    }

    // Operations that are only valid in `Final`.
    impl HttpResponse<Final> {
        // getter.
        pub fn get_headers(&self) -> &Vec<(String, String)> {
            &self.state.headers
        }

        // getter.
        pub fn get_body(&self) -> &str {
            &self.state.body
        }

        // getter.
        pub fn get_response_code(&self) -> u8 {
            self.state.response_code
        }

        // getter.
        pub fn get_status_line(&self) -> &str {
            &self.state.status_line
        }
    }
}

When you run the code using cargo run --bin ex3, it should produce the following output.

$ cargo run --bin ex3
Start state
response: HttpResponse {
    state: Start,
}
response size: 0 bytes
response: HttpResponse {
    state: HeaderAndBody {
        response_code: 200,
        status_line: "HTTP/1.1 200 OK",
        headers: None,
        body: None,
    },
}
HeaderAndBody state
response_code: 200
response body: None
response: HttpResponse {
    state: HeaderAndBody {
        response_code: 200,
        status_line: "HTTP/1.1 200 OK",
        headers: None,
        body: None,
    },
}
response size: 80 bytes
HeaderAndBody state # 2
response: HttpResponse {
    state: HeaderAndBody {
        response_code: 200,
        status_line: "HTTP/1.1 200 OK",
        headers: Some(
            [
                (
                    "Content-Type",
                    "text/html",
                ),
            ],
        ),
        body: Some(
            "<html><body>Hello World!</body></html>",
        ),
    },
}
response size: 80 bytes
Final state
response_code: 200
status_line: HTTP/1.1 200 OK
headers: [
    (
        "Content-Type",
        "text/html",
    ),
]
body: <html><body>Hello World!</body></html>
response: HttpResponse {
    state: Final {
        response_code: 200,
        status_line: "HTTP/1.1 200 OK",
        headers: [
            (
                "Content-Type",
                "text/html",
            ),
        ],
        body: "<html><body>Hello World!</body></html>",
    },
}
response size: 80 bytes

Example 3.1: Using enum and PhantomData instead of struct #

  • You can use enums instead of structs if you have shared data (inner) that you move with state transitions.
  • And you have to use PhantomData here.

Add the following code to the src/ex3_1.rs file.

use self::type_state_builder::HttpResponse;
use crossterm::style::Stylize;

pub fn main() -> Result<(), String> {
    let response = HttpResponse::<()>::new();
    println!("{}", "Start state".red().bold().underlined());
    println!("response: {:#?}", response);
    println!(
        "response size: {}",
        response.get_size().to_string().blue().bold()
    );

    // Status line is required.
    println!("{}", "HeaderAndBody state".red().bold().underlined());
    let mut response = response.set_status_line(200, "OK");
    println!("response_code: {}", response.get_response_code());
    println!("response body: {:#?}", response.get_body());
    println!("response: {:#?}", response);
    println!(
        "response size: {}",
        response.get_size().to_string().blue().bold()
    );

    // Body and headers are optional.
    println!("{}", "HeaderAndBody state # 2".red().bold().underlined());
    response.add_header("Content-Type", "text/html");
    response.set_body("<html><body>Hello World!</body></html>");
    println!("response: {:#?}", response);
    println!(
        "response size: {}",
        response.get_size().to_string().blue().bold()
    );

    // Final state.
    println!("{}", "Final state".red().bold().underlined());
    let response = response.finish();
    println!("response_code: {}", response.get_response_code());
    println!("status_line: {}", response.get_status_line());
    println!("headers: {:#?}", response.get_headers());
    println!("body: {:#?}", response.get_body());
    println!("response: {:#?}", response);
    println!(
        "response size: {}",
        response.get_size().to_string().blue().bold()
    );

    Ok(())
}

Note that this main function is the same as the one in the previous example.

The following code will be different. We are adding a new data module.

pub mod data {
    #[derive(Debug, Clone, Default)]
    pub struct HttpResponseData {
        pub response_code: u8,
        pub status_line: String,
        pub headers: Option<Vec<(String, String)>>,
        pub body: Option<String>,
    }
}

Here’s the new state module. Note the use of enums and PhantomData instead of structs.

pub mod state {
    #[derive(Debug, Clone)]
    pub enum Start {}

    #[derive(Debug, Clone)]
    pub enum HeaderAndBody {}

    #[derive(Debug, Clone)]
    pub struct Final {}

    // The following marker trait is used to restrict the operations
    // that are available in each state. This isn't strictly necessary,
    // but it's a nice thing to use in a where clause to restrict types.
    pub trait Marker {}
    impl Marker for () {}
    impl Marker for Start {}
    impl Marker for HeaderAndBody {}
    impl Marker for Final {}
}

Here is the changed code for the HttpResponse struct.

pub mod type_state_builder {
    use super::{
        data::HttpResponseData,
        state::{Final, HeaderAndBody, Marker, Start},
    };
    use std::marker::PhantomData;

    #[derive(Debug, Clone)]
    pub struct HttpResponse<S: Marker> {
        pub data: HttpResponseData,
        pub state: PhantomData<S>,
    }

    // Operations that are only valid in ().
    impl HttpResponse<()> {
        pub fn new() -> HttpResponse<Start> {
            HttpResponse {
                data: HttpResponseData::default(),
                state: PhantomData::<Start>,
            }
        }
    }

    // Operations that are only valid in Start.
    impl HttpResponse<Start> {
        // setter.
        pub fn set_status_line(
            self,
            response_code: u8,
            message: &str,
        ) -> HttpResponse<HeaderAndBody> {
            HttpResponse {
                data: {
                    let mut data = self.data;
                    data.response_code = response_code;
                    data.status_line = format!(
                        "HTTP/1.1 {} {}", response_code, message
                    );
                    data
                },
                state: PhantomData::<HeaderAndBody>,
            }
        }
    }

    // Operations that are only valid in HeaderAndBodyState.
    impl HttpResponse<HeaderAndBody> {
        // setter.
        pub fn add_header(&mut self, key: &str, value: &str) {
            let mut_data = &mut self.data;
            if mut_data.headers.is_none() {
                mut_data.headers.replace(Vec::new());
            }
            if let Some(headers) = mut_data.headers.as_mut() {
                headers.push((key.to_string(), value.to_string()))
            }
        }

        // getter.
        pub fn get_response_code(&self) -> u8 {
            self.data.response_code
        }

        // setter.
        pub fn set_body(&mut self, body: &str) {
            self.data.body.replace(body.to_string());
        }

        // getter.
        pub fn get_body(&self) -> Option<&str> {
            self.data.body.as_deref()
        }

        // transition to Final state.
        pub fn finish(self) -> HttpResponse<Final> {
            let mut data = self.data;
            HttpResponse {
                data: HttpResponseData {
                    response_code: data.response_code,
                    status_line: data.status_line.clone(),
                    headers: Some(data.headers.take().unwrap_or_default()),
                    body: Some(data.body.take().unwrap_or_default()),
                },
                state: PhantomData::<Final>,
            }
        }
    }

    // Operations that are only valid in FinalState.
    impl HttpResponse<Final> {
        pub fn get_headers(&self) -> &Option<Vec<(String, String)>> {
            &self.data.headers
        }

        pub fn get_body(&self) -> &Option<String> {
            &self.data.body
        }

        pub fn get_response_code(&self) -> u8 {
            self.data.response_code
        }

        pub fn get_status_line(&self) -> &str {
            &self.data.status_line
        }
    }

    // Operations that are available in all states.
    impl<S> HttpResponse<S>
    where
        S: Marker,
    {
        pub fn get_size(&self) -> String {
            let len = std::mem::size_of_val(self);
            format!("{} bytes", len)
        }
    }
}

Here’s the output when you run the code using cargo run --bin ex3_1.

$ cargo run --bin ex3_1
Start state
response: HttpResponse {
    data: HttpResponseData {
        response_code: 0,
        status_line: "",
        headers: None,
        body: None,
    },
    state: PhantomData<ex3_1::state::Start>,
}
response size: 80 bytes
HeaderAndBody state
response_code: 200
response body: None
response: HttpResponse {
    data: HttpResponseData {
        response_code: 200,
        status_line: "HTTP/1.1 200 OK",
        headers: None,
        body: None,
    },
    state: PhantomData<ex3_1::state::HeaderAndBody>,
}
response size: 80 bytes
HeaderAndBody state # 2
response: HttpResponse {
    data: HttpResponseData {
        response_code: 200,
        status_line: "HTTP/1.1 200 OK",
        headers: Some(
            [
                (
                    "Content-Type",
                    "text/html",
                ),
            ],
        ),
        body: Some(
            "<html><body>Hello World!</body></html>",
        ),
    },
    state: PhantomData<ex3_1::state::HeaderAndBody>,
}
response size: 80 bytes
Final state
response_code: 200
status_line: HTTP/1.1 200 OK
headers: Some(
    [
        (
            "Content-Type",
            "text/html",
        ),
    ],
)
body: Some(
    "<html><body>Hello World!</body></html>",
)
response: HttpResponse {
    data: HttpResponseData {
        response_code: 200,
        status_line: "HTTP/1.1 200 OK",
        headers: Some(
            [
                (
                    "Content-Type",
                    "text/html",
                ),
            ],
        ),
        body: Some(
            "<html><body>Hello World!</body></html>",
        ),
    },
    state: PhantomData<ex3_1::state::Final>,
}
response size: 80 bytes

Parting thoughts #

To get an experiential understanding of the typestate pattern, you should try to build something using it. It’s a powerful pattern that can help you write more robust and predictable code. And it’s a great way to leverage Rust’s type system to enforce state transitions in your code. I encourage you to clone the repo and run the code to see how it works. And make changes to it to see if you can make it behave differently and use it in your own projects.

Build with Naz video series on developerlife.com YouTube channel #

You can watch a video series on building this crate with Naz on the developerlife.com YouTube channel.

📦 Install our useful Rust command line apps using cargo install r3bl-cmdr (they are from the r3bl-open-core project):
  • 🐱giti: run interactive git commands with confidence in your terminal
  • 🦜edi: edit Markdown with style in your terminal

giti in action

edi in action

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