Either -> Result

[janekx21]

Either is the wrong name for this monad, because Either = Left l | Right r sound symmetrical. But Either is not symmetrical, because its mapping and bind only works on the Right r constructor. The proper name should be Result = Ok value | Err error. This is not symmetrical and easier to read.

Elm Reference: https://package.elm-lang.org/packages/elm/core/latest/Result

[apfohl]

To some degree you are right. But they are two different things when it comes to semantics. Result<T,TError> is basically a specialized variant of the Either<TLeft,TRight> monad, with the intention of getting an actual result in the success case and an error object in the case of failure.

Either<TLeft,TRight> can be used for this application as well, because it obviously has two sides. But it lacks the semantics. Mapping and binding works for both sides. If you see the ‚right‘ side as the correct one (as the name implies) you have the methods Map and Bind like in Maybe. For ‚left‘ you have MapLeft and MapRight. So it‘s fully usable for both cases. You would have a similar set of methods for a Result<T,TError> implementation.

Speaking of Result<T,TError>, I was planning this monad basically as the next addition to this library. If you wish, we can share knowledge and implement it. There is also another similar/same monad called ‚Exceptional‘.

hope this helps a little bit, with the understanding, why we only have Either<TLeft,TRight> at the moment. And thanks the the thoughts.

[apfohl]

@janekx21 We discussed this topic and considering a rewrite of the Either monad into Result. We will keep the Either monad as obsolete in the library to maintain backwards compatability.

[Dirk-Peters-BF]

The whole package will most likely be obsolete with the next dotnet LTS version, because actual discriminated unions, including built in Option and Result types, will be added.

[apfohl]

Is it already in the next LTS? You have a link? Only saw the proposal. But yes. It’s a good thing that this is obsolete soon.

[Dirk-Peters-BF]

correction: "maybe obsolete with the next dotnet LTS"
IMHO the proposal for typed unions looks already quite settled