#![feature(unboxed_closures, fn_traits)]

use std::future::Future;

fn main() {
    let mut conn = Connection;
    conn.transaction(|conn| async move {
        conn.execute("Hi").await?;
        conn.execute("Hi2").await
    });
}

struct Connection;

impl Connection {
    async fn execute(&mut self, s: &str) -> Result<(), ()> {
        println!("{}", s);
        Ok(())
    }

    async fn transaction<F, T>(&mut self, f: F) -> Result<T, ()>
    where
        for<'a> F: FnOnce<(&'a mut Self,)>,
        for<'a> <F as FnOnce<(&'a mut Self,)>>::Output: Future<Output = Result<T, ()>>,
    {
        self.execute("BEGIN").await?;
        let res = f(self).await;
        if res.is_ok() {
            self.execute("COMMIT").await?;
        } else {
            self.execute("ROLLBACK").await?;
        }
        res
    }
}