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

[Greg Titus]

> On Jan 26, 2016, at 9:30 AM, Developer via swift-evolution <swift-evolution at swift.org> wrote:
> 
>> - occasionally: a rather poorly-named, 2-way sum type (poorly-named b/c its convention won’t generalize to 3-way sums, or 4-ways sums, etc.)
> 
> Good thing Either<Either<...>, Either<...>> works.

This is another example of exactly why Either is terrible in practice. I understand that from a type algebra point of view this is perfectly reasonable and clean and that a 2-way sum is all that is necessary because larger sums are easily created by construction. Type theory-wise it’s lovely.

But from a literate programming point of view, when will "case let .Left(.Left(a)): case let .Left(.Right(b)): case let .Right(.Left(c)): case let .Right(.Right(d)): “ ever be the most understandable way to express _anything_ in Swift? And why would we want to encourage such things?

	- Greg
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