Code shared from the Rust Playground

playground.rs

//! この方法の方でCakeパターンをするとintellij-rustでの補完が効くケースが多い。

type Error = Box<std::error::Error>;

#[derive(Debug, Clone, Copy)]
struct EmployeeId(u64);

#[derive(Debug)]
struct Employee {
    id: EmployeeId,
    name: String,
}

trait DataStore {
    fn get_employee_by_id(&self, id: EmployeeId) -> Result<Employee, Error>;
}

trait HaveDataStore<D: DataStore> {
    fn dao(&self) -> &D;
}

trait Repository<D: DataStore> {
    fn get_employee(&self, id: EmployeeId) -> Result<Employee, Error>;
    fn does_employee_exist(&self, id: EmployeeId) -> Result<bool, Error>;
}

impl<T, D> Repository<D> for T
where
    D: DataStore,
    T: HaveDataStore<D>,
{
    fn get_employee(&self, id: EmployeeId) -> Result<Employee, Error> {
        self.dao().get_employee_by_id(id)
    }

    fn does_employee_exist(&self, id: EmployeeId) -> Result<bool, Error> {
        self.get_employee(id).map(|_| true)
    }
}

#[derive(Debug, Default)]
struct Dao;

impl DataStore for Dao {
    fn get_employee_by_id(&self, _: EmployeeId) -> Result<Employee, Error> {
        Ok(Employee {
            id: EmployeeId(1),
            name: "ほげ 太郎".to_string(),
        })
    }
}

#[derive(Debug)]
struct Dao2;

impl DataStore for Dao2 {
    fn get_employee_by_id(&self, _: EmployeeId) -> Result<Employee, Error> {
        Ok(Employee {
            id: EmployeeId(2),
            name: "ほげ 太郎２".to_string(),
        })
    }
}

#[derive(Debug)]
struct MyRepository<D>(D);

impl<D: DataStore> MyRepository<D> {
    fn new(dao: D) -> MyRepository<D> {
        MyRepository(dao)
    }
}

impl<D: DataStore> HaveDataStore<D> for MyRepository<D> {
    fn dao(&self) -> &D {
        &self.0
    }
}

fn main() {
    let r = MyRepository::new(Dao);
    let id = EmployeeId(1);
    println!("{:?}", r.get_employee(id).unwrap());
    println!("{:?}", r.does_employee_exist(id).unwrap());
    
    let r = MyRepository::new(Dao2);
    let id = EmployeeId(1);
    println!("{:?}", r.get_employee(id).unwrap());
    println!("{:?}", r.does_employee_exist(id).unwrap());
}