Statum

Statum is a zero-boilerplate library for finite-state machines in Rust, with compile-time state transition validation. It provides two attribute macros:

• #[state] for defining states (as enums).
• #[machine] for creating a state machine struct that tracks which state you’re in at compile time.

Quick Start (Minimal Example)

Here’s the simplest usage of Statum without any extra features:

use statum::{state, machine};

// 1. Define your states as an enum.
#[state]
pub enum LightState {
    Off,
    On,
}

// 2. Create a machine struct that references one of those states.
#[machine]
pub struct Light<S: LightState> {
    name: String,
}

// 3. Implement transitions for each state.
impl Light<Off> {
    pub fn switch_on(self) -> Light<On> {
        self.transition()
    }
}

impl Light<On> {
    pub fn switch_off(self) -> Light<Off> {
        self.transition()
    }
}

fn main() {
    // 4. Create a machine with the "Off" state.
    let light = Light::<Off>::new("desk lamp".to_owned());

    // 5. Transition from Off -> On, On -> Off, etc.
    let light = light.switch_on();
    let light = light.switch_off();
}

How It Works

• #[state] transforms your enum, generating one struct per variant (like Off and On), plus a trait LightState.
• #[machine] injects extra fields (marker, state_data) to track which state you’re in, letting you define transitions that change the state at the type level.

That’s it! You now have a compile-time guaranteed state machine where invalid transitions are impossible.

---

Additional Features & Examples

1. Adding Debug, Clone, or Other Derives

By default, you can add normal Rust derives on your enum and struct. For example:

#[state]
#[derive(Debug, Clone)]
pub enum LightState {
    Off,
    On,
}

#[machine]
#[derive(Debug, Clone)]
pub struct Light<S: LightState> {
    name: String,
}

Important: If you place #[derive(...)] above #[machine], you may see an error like:

error[E0063]: missing fields `marker` and `state_data` in initializer of `Light<_>`
   |
14 | #[derive(Debug, Clone)]
   |          ^ missing `marker` and `state_data`

That’s because the derive macro for Clone, Debug, etc., expands before #[machine] has injected these extra fields. To avoid this, either:

• Put #[machine] above the derive(s), or
• Remove the conflicting derive(s) from the same item.

For example, this works:

#[machine]
#[derive(Debug, Clone)]
pub struct Light<S: LightState> {
    name: String,
}

---

2. serde Integration

Statum can optionally propagate Serialize/Deserialize derives if you enable the "serde" feature and derive those on your #[state] enum. For example:

[dependencies]
statum = { version = "x.y.z", features = ["serde"] }
serde = { version = "1.0", features = ["derive"] }

Then, in your code:

use statum::state;

#[state]
#[derive(Debug, serde::Serialize, serde::Deserialize)]
pub enum DocumentState {
    Draft,
    Published,
}

If you enable Statum’s "serde" feature, any #[derive(Serialize)] and #[derive(Deserialize)] you put on the enum will get passed through to the expanded variant structs. If you do not enable that feature, deriving those traits will likely fail to compile.

---

3. Complex Transitions & Data-Bearing States

States can hold data. For example:

#[state]
pub enum ReviewState {
    Draft,
    InReview(ReviewData),
    Published,
}

#[derive(Debug)]
pub struct ReviewData {
    reviewer: String,
    notes: Vec<String>,
}

#[machine]
pub struct Document<S: ReviewState> {
    id: String,
    content: String,
}

// ...

impl Document<Draft> {
    pub fn submit_for_review(self, reviewer: String) -> Document<InReview> {
        let data = ReviewData { reviewer, notes: vec![] };
        self.transition_with(data)
    }
}

// ...

Accessing State Data

Use .get_state_data() or .get_state_data_mut() to interact with the state-specific data:

impl Document<Review> {
    fn add_comment(&mut self, comment: String) {
        if let Some(review_data) = self.get_state_data_mut() {
            review_data.comments.push(comment);
        }
    }

    fn reviewer_name(&self) -> Option<&str> {
        self.get_state_data().map(|data| data.reviewer.as_str())
    }

    fn approve(self) -> Document<Published> {
        self.transition()
    }
}

---

4. Attribute Ordering

• #[state] must go on an enum.
• #[machine] must go on a struct.
• Because #[machine] injects extra fields, you need it above any user #[derive(...)]. If you place #[derive(...) ] first, you might see “missing fields marker and state_data in initializer” errors.

---

5. Implementing the Typestate Builder Pattern with Statum

The typestate builder pattern is a powerful way to enforce correct usage of a sequence of steps at compile time. With statum, you can implement this pattern using the provided #[state] and #[machine] macros to ensure type-safe state transitions in your builders.

This guide will walk you through implementing a typestate builder for a hypothetical "User Registration" workflow.

---

Overview

Imagine we have a multi-step process for registering a user:

1. Collect the user's name.
2. Set the user's email.
3. Submit the registration.

Using the typestate builder pattern, we can ensure:

• Each step must be completed before moving to the next.
• Skipping steps or submitting prematurely results in compile-time errors.

---

Steps to Implement

1. Define States

Each step in the builder process is represented as a state using #[state]. For example:

use statum::{state, machine};

#[state]
pub enum UserState {
    NameNotSet,
    NameSet(NameData),
    EmailSet(UserData),
}

#[derive(Debug, Clone)]
pub struct NameData {
    name: String,
}

#[derive(Debug, Clone)]
pub struct UserData {
    name: String,
    email: String,
}

Here:

• NameNotSet: The initial state where the name has not been set.
• NameSet: The state where the name is provided but the email is not.
• EmailSet: The final state before submission.

2. Create the Builder Machine

The builder itself is a #[machine]-decorated struct that uses the defined states.

#[machine]
#[derive(Debug, Clone)]
pub struct UserBuilder<S: UserState> {
    id: u32,
}

This struct will manage transitions between states.

3. Define State Transitions

Implement methods to move from one state to the next:

Transition from NameNotSet to NameSet

impl UserBuilder<NameNotSet> {
    pub fn set_name(self, name: String) -> UserBuilder<NameSet> {
        let data = NameData { name };
        self.transition_with(data)
    }
}

Transition from NameSet to EmailSet

impl UserBuilder<NameSet> {
    pub fn set_email(self, email: String) -> UserBuilder<EmailSet> {
        let NameData { name } = self.get_state_data().unwrap().clone();
        let data = UserData { name, email };
        self.transition_with(data)
    }
}

Transition from EmailSet to Submission

impl UserBuilder<EmailSet> {
    pub fn submit(self) -> Result<(), &'static str> {
        let UserData { name, email } = self.get_state_data().unwrap();
        println!("User registered: Name = {}, Email = {}", name, email);
        Ok(())
    }
}

4. Example Usage

Here’s how you would use this builder:

fn main() {
    let builder = UserBuilder::<NameNotSet>::new(1);

    // Step 1: Set the name
    let builder = builder.set_name("Alice".to_string());

    // Step 2: Set the email
    let builder = builder.set_email("alice@example.com".to_string());

    // Step 3: Submit the registration
    builder.submit().unwrap();
}

Compile-Time Guarantees

• You cannot set the email without first setting the name:

  let builder = UserBuilder::<NameNotSet>::new(1);
  let builder = builder.set_email("alice@example.com".to_string()); // Compile-time error
  

• You cannot submit the builder without setting the name and email:

  let builder = UserBuilder::<NameNotSet>::new(1);
  builder.submit(); // Compile-time error
  

Common Errors and Tips

1. missing fields marker and state_data

   • Usually means your derive macros (e.g., Clone or Debug) expanded before Statum could inject those fields. Move #[machine] above your derives, or remove them.

2. cannot find type X in this scope

   • Ensure that you define your #[machine] struct before you reference it in impl blocks or function calls.

3. Feature gating

   • If you’re using #[derive(Serialize, Deserialize)] on a #[state] enum but didn’t enable the serde feature in Statum, you’ll get compile errors about missing trait bounds.

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Lint Warnings (unexpected_cfgs)

If you see warnings like:

= note: no expected values for `feature`
= help: consider adding `foo` as a feature in `Cargo.toml`

it means you have the unexpected_cfgs lint enabled but you haven’t told your crate “feature = foo” is valid. This is a Rust nightly lint that ensures you only use #[cfg(feature="...")] with known feature values.

To fix it, either disable the lint or declare the allowed values in your crate’s Cargo.toml:

[lints.rust.unexpected_cfgs]
check-cfg = [
  'cfg(feature, values("serde"))'
]
level = "warn"

License

Statum is distributed under the terms of the MIT license. See LICENSE for details.