# Result type auto error type coersion **URL:** \ **Category:** language design\ **Created:** [December 9, 2022, 3:06pm UTC](https://internals.rust-lang.org/t/result-type-auto-error-type-coersion/17941 "2022-12-09T15:06:57Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![RedIODev](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/rediodev/32/10233_2.png) [@RedIODev](https://internals.rust-lang.org/u/RedIODev)\ **Post date:** [December 9, 2022, 3:06pm UTC](https://internals.rust-lang.org/t/result-type-auto-error-type-coersion/17941/1 "2022-12-09T15:06:57Z")
## The problem: If you use some libraries every library brings its own error type. These error types then lie outside of your crate therefore you can't coerce them with the ? operator in one function scope. You therefore then create your own enum type or struct that then implements From for all the error types (and of course std::error::Error) The problem is when you simply want to return a result: ```rust fn foo() -> Result { lib1::foo()?; lib2::foo() //Error different error types } ``` ## Solutions One solution is to call lib2::foo().map\_error(MyError::From). But this is very wordy and could be very well done by the compiler. We just need a way to tell the compiler that he needs to use the From implementation. My current solution is an extention function for Result: ```rust fn auto_err(self) -> Result where F: From; ``` This helps a lot to reduce the boilerplate of obvious from into conversions. It might even be worth a keyword but that's debatable. When it's chosen to get a keyword I would suggest the postfix ! The sample code above with the operator would look like this: ```rust fn foo() -> Result { lib1::foo()?; lib2::foo()! } ``` It might be beneficial to from/into all generic parameters of the type operated on (in this case Result). ---
**Author:** ![nakacristo](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/nakacristo/32/10249_2.png) [@nakacristo](https://internals.rust-lang.org/u/nakacristo)\ **Post date:** [December 9, 2022, 3:28pm UTC](https://internals.rust-lang.org/t/result-type-auto-error-type-coersion/17941/2 "2022-12-09T15:28:07Z")
Is there any problem with this? ```rust fn foo() -> Result { lib1::foo()?; Ok(lib2::foo()?) } ``` ---
**Author:** ![scottmcm](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/scottmcm/32/2355_2.png) [@scottmcm](https://internals.rust-lang.org/u/scottmcm)\ **Post date:** [December 9, 2022, 5:37pm UTC](https://internals.rust-lang.org/t/result-type-auto-error-type-coersion/17941/3 "2022-12-09T17:37:58Z")
> [@RedIODev](#): > > ```rust > fn foo() -> Result { > lib1::foo()?; > lib2::foo() //Error different error types > } > > ``` I think the best way forward for this is the "you put `?` on everything fallible" pattern (perhaps plus `try` blocks). Because the best answer is not "make the last one even more special with another operator", but "make the last one the same as the others": ```rust fn foo() -> Result { try { lib1::foo()?; // it's fallible so it's `?`'d lib2::foo()?; // it's fallible so it's `?`'d lib3::foo()? // it's fallible so it's `?`'d } } ``` On stable today you can write that as ```rust fn foo() -> Result { Ok({ lib1::foo()?; // it's fallible so it's `?`'d lib2::foo()?; // it's fallible so it's `?`'d lib3::foo()? // it's fallible so it's `?`'d }) } ``` ---
**Author:** ![RedIODev](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/rediodev/32/10233_2.png) [@RedIODev](https://internals.rust-lang.org/u/RedIODev)\ **Post date:** [December 10, 2022, 8:24pm UTC](https://internals.rust-lang.org/t/result-type-auto-error-type-coersion/17941/5 "2022-12-10T20:24:22Z")
I think after some afterthought it simply lacks a From implementation for result. Something like this: ```rust impl From> for Result where U: From, F: From { fn from(source: Result) -> Self { source.map(U::from).map_err(F::from) } } ``` This would allow for: ```rust fn foo() -> Result { lib1::foo()?; lib2::foo().into() } ``` Which is in my opinion pretty reasonable. ---
**Author:** ![scottmcm](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/scottmcm/32/2355_2.png) [@scottmcm](https://internals.rust-lang.org/u/scottmcm)\ **Post date:** [December 10, 2022, 8:43pm UTC](https://internals.rust-lang.org/t/result-type-auto-error-type-coersion/17941/6 "2022-12-10T20:43:24Z")
> [@RedIODev](#): > > Which is in my opinion pretty reasonable. I agree, but it can't be written today because it overlaps with the identity impl. Would be nice, though! ---
**Author:** ![nakacristo](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/nakacristo/32/10249_2.png) [@nakacristo](https://internals.rust-lang.org/u/nakacristo)\ **Post date:** [December 10, 2022, 8:43pm UTC](https://internals.rust-lang.org/t/result-type-auto-error-type-coersion/17941/7 "2022-12-10T20:43:54Z")
That could be nice, but it conflicts with the core `impl From for T;` The possibility of a helper method was tried in [rfc 1996](https://github.com/rust-lang/rfcs/pull/1996) but finally discarded because of lack of motivation in the success conversion. In that RFC thread @withoutboats comments about the possibility of implementing the trait: > Someday it will work (because we want intersection impl specialization as soon as its feasible to implement it), and the impl should exist so that you can use it generically. That was in 2017, I have no idea what that "someday" would be. ---
**Author:** ![system](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/system/32/14092_2.png) [@system](https://internals.rust-lang.org/u/system)\ **Post date:** [March 10, 2023, 8:44pm UTC](https://internals.rust-lang.org/t/result-type-auto-error-type-coersion/17941/8 "2023-03-10T20:44:24Z")
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