Illustrating rust-lang/rfcs#2261

cargo.out

@@ -1,14 +1,20 @@
-   Compiling futuretest v0.1.0 (file:///mnt/c/Users/Mahmoud/git/futuretest)
-error[E0277]: the trait bound `(): futures::Future` is not satisfied
-  --> src/main.rs:17:1
+   Compiling futuretest v0.1.0 (file:///mnt/d/GIT/futuretest)
+error[E0308]: match arms have incompatible types
+  --> src/main.rs:21:13
    |
-17 | / fn test() -> MapErr<(), ()> {
-18 | |     future::ok(())
-19 | |         .map_err(|String| ())
-20 | | }
-   | |_^ the trait `futures::Future` is not implemented for `()`
+21 | /             match err {
+22 | |                 ErrorCode::Case1 => case1(),
+23 | |                 ErrorCode::Case2 => case2(),
+24 | |             }
+   | |_____________^ expected anonymized type, found a different anonymized type
    |
-   = note: required by `futures::MapErr`
+   = note: expected type `impl futures::Future` (anonymized type)
+              found type `impl futures::Future` (anonymized type)
+note: match arm with an incompatible type
+  --> src/main.rs:23:37
+   |
+23 |                 ErrorCode::Case2 => case2(),
+   |                                     ^^^^^^^
 
 error: aborting due to previous error
 

src/main.rs

@@ -6,15 +6,37 @@ use futures::future::{self};
 use futures::future::*;
 use tokio_core::reactor::Core;
 
+enum ErrorCode
+{
+    Case1,
+    Case2,
+}
+
 fn main() {
     let mut core = Core::new().expect("Failed to initialize tokio_core reactor!");
 
-    let f = test();
+    let f = async_entry_point()
+        .or_else(move |err| {
+            //the problem is that the three matches below resolve to different anonymized types
+            match err {
+                ErrorCode::Case1 => case1(),
+                ErrorCode::Case2 => case2(),
+            }
+        })
+    ;
 
-    core.run(f).unwrap();
+    core.run(f);
+}
+
+fn async_entry_point() -> impl Future<Item=(), Error=ErrorCode> {
+    future::ok(())
 }
 
-fn test() -> MapErr<(), ()> {
+fn case1() -> impl Future<Item=(), Error=ErrorCode> {
     future::ok(())
-        .map_err(|String| ())
 }
+
+fn case2() -> impl Future<Item=(), Error=ErrorCode> {
+    future::ok(())
+}
+