[swift-evolution] [proposal] Either in the Swift Standard Library

Craig Cruden ccruden at novafore.com
Tue Jan 26 00:13:17 CST 2016

I don’t agree that `Result` as a name is any more intuitive than `Either` (a disjointed union).  The end result of any function is a `resulting` value regardless of what type it is.  `Either` is just indicating that the function can return either of two types (generalized).  So when you are returning either of two types (regardless of whether it is validation) - it would intuitively be driven by what type you are returning.  I have never seen an Either used for the same type on both sides.  As such I don’t find `Result` being intuitive vs `Either` being not a very strong argument.  

`Either` is a type that is common to many functional languages, or functional implementations in more general languages.

> On 2016-01-26, at 12:55:27, Kevin Ballard via swift-evolution <swift-evolution at swift.org> wrote:
> There absolutely is a cost. `Result<T>` has a rather intuitive meaning. `Either<T>` has no intuitive meaning whatsoever. It says absolutely nothing about what it means beyond the fact that there are two potential values. As a result, it is a largely useless type whose sole redeeming feature is it allows developers to avoid having to define their own enum, but in most cases that aren't covered by Result<T> you actually want to define your own enum so you can attach meaning to the value.
> If it's not obvious, I'm very strongly against having a generic Either type, but I do want a Result<T> or Result<T,E>.
> -Kevin Ballard
> On Fri, Jan 22, 2016, at 10:22 PM, Developer via swift-evolution wrote:
>> My overwhelming concern, after having a conversation with Chris, is that implementing a Result<T> means we are strongly implying a particular semantics and use case when we could generalize and abstract for no cost but an extra generic parameter.  In F#, Core.Choice can be used to build a Validation or Result monad, but the converse is impossible.
>> ~Robert Widmann
>> 2016/01/23 1:05、Rob Mayoff via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> のメッセージ:
>>> Just added a section of motivating examples to the Either proposal.  Ping me if you have any more that I missed ('cause I'm sure I did miss a lot).
>>> https://github.com/typelift/swift-evolution/blob/either-or/proposals/0024-either.md#motivating-examples <https://github.com/typelift/swift-evolution/blob/either-or/proposals/0024-either.md#motivating-examples>
>>> Your motivating examples (including all the projects you linked except "Any many more") overwhelmingly use the Either (or similar type) to represent success/failure. I'm not sure there's a single example where the names Left and Right actually make sense in the problem domain. I'm not 100% sure about func alternate in Madness/Alternation.swift. It definitely uses Left/Right to mean Failure/Result, but I couldn't tell if it also uses them as something else. Which makes those names all the more maddening.
>>> I checked my company's largest Scala project, which is over 300,000 lines. We use Scala's Try/Success/Failure in dozens of places. We use Either/Left/Right once, in a thrown-together report-generating script, which would probably have been written in awk or perl if it didn't need to read binary log files. (The ability of IntelliJ to reliably find all uses of a class or method is not to be underestimated. Hint hint, team Xcode.)
>>> I think a Result/Success/Failure type is warranted, but I'm very skeptical about generic Either/Left/Right.
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