# 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:
```rust
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.
// The fields in this struct are intended to provide context (e.g., configurations, clients, or dependencies)
// needed across different states of the machine. This is similar to how Axum's `with_state`
// shares context in routers. If you need to include data relevant to a specific state,
// it is better to store that data within the state itself (as we discuss later in the README).
#[machine]
pub struct Light<S: LightState> {
name: String, // Contextual fields go here, like configurations, an identifier, clients, etc.
}
// 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.
// note: you dont need the ::<Off> here, it is inferred
// but it is shown here for clarity
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
statum is in active development so if you need something else let us know!
Supported derives:
- Serialize (with serde feature enabled)
- Deserialize (with serde feature enabled)
- Debug
- Clone
- Default
- Eq
- PartialEq
- Hash
- PartialOrd
- Ord
- Copy
By default, you can add normal Rust derives on your enum and struct. For example:
```rust
#[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<_>`
|
```
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:
```rust
#[machine]
#[derive(Debug, Clone)]
pub struct Light<S: LightState> {
name: String,
}
```
This does not:
```rust
#[derive(Debug, Clone)] //note the position of the derive
#[machine]
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:
```toml
[dependencies]
statum = { version = "x.y.z", features = ["serde"] }
serde = { version = "1.0", features = ["derive"] }
```
Then, in your code:
```rust
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:
```rust
#[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:
```rust
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:
```rust
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.
```rust
#[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`
```rust
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`
```rust
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
```rust
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:
```rust
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:
```rust
let builder = UserBuilder::<NameNotSet>::new(1);
let builder = builder.set_email("alice@example.com".to_string()); ```
- You **cannot** submit the builder without setting the name and email:
```rust
let builder = UserBuilder::<NameNotSet>::new(1);
builder.submit(); ```
## 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.
---
## 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`:
```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](LICENSE) for details.