Illustrating alexcrichton/futures-rs#683

cargo.out

@@ -1,14 +1,22 @@
-   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:28:13
    |
-17 | / fn test() -> MapErr<(), ()> {
-18 | |     future::ok(())
-19 | |         .map_err(|String| ())
-20 | | }
-   | |_^ the trait `futures::Future` is not implemented for `()`
+28 | /             match err {
+29 | |                 ErrorCode::CanHandleWithAsync => async_error_handler()
+30 | |                     .map_err(|_| ErrorCode::CannotHandle).wait(),
+31 | |                 ErrorCode::CanHandleWithSync => future::result(sync_error_handler()),
+32 | |                 ErrorCode::CannotHandle => future::err(ErrorCode::CannotHandle),
+33 | |             }
+   | |_____________^ expected enum `std::result::Result`, found struct `futures::FutureResult`
    |
-   = note: required by `futures::MapErr`
+   = note: expected type `std::result::Result<(), ErrorCode>`
+              found type `futures::FutureResult<(), ErrorCode>`
+note: match arm with an incompatible type
+  --> src/main.rs:31:49
+   |
+31 |                 ErrorCode::CanHandleWithSync => future::result(sync_error_handler()),
+   |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error
 

src/main.rs

@@ -2,19 +2,50 @@
 
 extern crate futures;
 extern crate tokio_core;
-use futures::future::{self};
+use futures::future::{self, FutureResult};
 use futures::future::*;
 use tokio_core::reactor::Core;
 
+#[derive(Debug)]
+enum ErrorCode
+{
+    CanHandleWithAsync,
+    CanHandleWithSync,
+    CannotHandle
+}
+
+#[derive(Debug)]
+enum ErrorCode2
+{
+}
+
 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 future types
+            match err {
+                ErrorCode::CanHandleWithAsync => async_error_handler()
+                    .map_err(|_| ErrorCode::CannotHandle).wait(),
+                ErrorCode::CanHandleWithSync => future::result(sync_error_handler()),
+                ErrorCode::CannotHandle => future::err(ErrorCode::CannotHandle),
+            }
+        })
+    ;
 
     core.run(f).unwrap();
 }
 
-fn test() -> MapErr<(), ()> {
+fn async_entry_point() -> FutureResult<(), ErrorCode> {
     future::ok(())
-        .map_err(|String| ())
 }
+
+fn async_error_handler() -> FutureResult<(), ErrorCode2> {
+    future::ok(())
+}
+
+fn sync_error_handler() -> Result<(), ErrorCode> {
+    Ok(())
+}
+