dbofmmbt · February 24, 2021 16:16 · Feb 24, 2021
diff --git a/main.rs b/main.rs
@@ -0,0 +1,107 @@
+/// 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));
+    }
+}
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