/// This code was built on @scottmcm 's PR for the new Try trait design.

#![feature(try_trait_v2)]
#![feature(control_flow_enum)]
#![feature(never_type)]
#![allow(dead_code)]

use std::ops::{ControlFlow, FromResidual, Try};

enum MyFlow<T, S = ()> {
    Stay(S),
    Up(T),
}

struct MyFlowResidual<T>(T);

impl<T, S> Try for MyFlow<T, S> {
    type Ok = S;
    type Residual = MyFlowResidual<T>;

    fn from_output(v: <Self as Try>::Ok) -> Self {
        Self::Stay(v)
    }

    fn branch(self) -> ControlFlow<<Self as Try>::Residual, <Self as Try>::Ok> {
        match self {
            Self::Stay(it) => ControlFlow::Continue(it),
            Self::Up(payload) => ControlFlow::Break(MyFlowResidual(payload)),
        }
    }
}

impl<T, S> FromResidual for MyFlow<T, S> {
    fn from_residual(x: <Self as Try>::Residual) -> Self {
        MyFlow::Up(x.0.into())
    }
}

impl<T, E> FromResidual<MyFlowResidual<T>> for Result<T, E> {
    fn from_residual(r: MyFlowResidual<T>) -> Self {
        Ok(r.0)
    }
}

impl<T, E> FromResidual<MyFlow<T, !>> for Result<T, E> {
    fn from_residual(x: MyFlow<T, !>) -> Self {
        match x {
            MyFlow::Up(it) => Ok(it),
            // I thought that this match would be exaustive without this line below (because of `!` in x), but the compiler complained to me.
            MyFlow::Stay(_) => panic!(),
        }
    }
}

/// Early Exit for the happy case.
trait Lift {
    type Break;
    type Continue;

    fn lift(self) -> MyFlow<Self::Break, Self::Continue>;
}

impl<T, E> Lift for Result<T, E> {
    type Break = T;
    type Continue = E;

    fn lift(self) -> MyFlow<Self::Break, Self::Continue> {
        match self {
            Ok(t) => MyFlow::Up(t),
            Err(e) => MyFlow::Stay(e),
        }
    }
}

fn main() {
    println!("Hello, world!");
}

fn test() -> Result<usize, String> {
    a().lift()?;
    b().lift()?;
    c().lift()?;

    Err("end".to_string())
}

fn a() -> Result<usize, ()> {
    Err(())
}

fn b() -> Result<usize, ()> {
    Ok(42)
}

fn c() -> Result<usize, ()> {
    Ok(7)
}

#[cfg(test)]
mod tests {
    use crate::test;

    #[test]
    fn it_works() {
        assert_eq!(test(), Ok(42));
    }
}