# 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.
#[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:
```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,
}
```
---
### 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 generally need it _above_ any user `#[derive(...)]`. If you place `#[derive(...) ]` first, you might see “missing fields `marker` and `state_data` in initializer” errors.
---
## 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.