Typestate Pattern

Pattern สำหรับ compile-time state machine validation

Basic Typestate

// States as empty structs
struct Draft;
struct Review;
struct Published;

// Document with state type parameter
struct Document<State> {
    content: String,
    _state: std::marker::PhantomData<State>,
}

impl Document<Draft> {
    fn new(content: &str) -> Self {
        Document {
            content: content.to_string(),
            _state: std::marker::PhantomData,
        }
    }

    fn edit(&mut self, new_content: &str) {
        self.content = new_content.to_string();
    }

    // Transition to Review
    fn submit_for_review(self) -> Document<Review> {
        println!("Submitting for review...");
        Document {
            content: self.content,
            _state: std::marker::PhantomData,
        }
    }
}

impl Document<Review> {
    // Can only publish from Review state
    fn approve(self) -> Document<Published> {
        println!("Approved!");
        Document {
            content: self.content,
            _state: std::marker::PhantomData,
        }
    }

    fn reject(self) -> Document<Draft> {
        println!("Rejected, back to draft");
        Document {
            content: self.content,
            _state: std::marker::PhantomData,
        }
    }
}

impl Document<Published> {
    fn get_content(&self) -> &str {
        &self.content
    }
}

fn main() {
    let mut doc = Document::<Draft>::new("Initial content");
    doc.edit("Better content");

    let doc = doc.submit_for_review();
    // doc.edit("x");  // Error! Not in Draft state

    let doc = doc.approve();
    println!("Published: {}", doc.get_content());
}