Either type implementation for Swift.

Scala's Either inspires the development of this Either type for Swift.

Either represents a value of one of two possible types (a disjoint union). An instance of Either is an instance of either left or right.

A common use of Either is as an alternative to Optional for dealing with possibly missing values. In this usage, Optional.none is replaced with a Either.left, which can contain useful information; and a Either.right takes the place of Optional.some.

Another common use of Either is as an alternative to Result without strictly constraining the conformance to the Error protocol. In this usage, Result.failure is replaced with a Either.left; and a Either.right takes the place of Result.success.

NOTE: Convention dictates that Either.left is used for failure and Either.right is used for success - but this IS NOT mandatory.

A pretty simple example of possible use for Either is a function that may fail or produce a result.

For example, let's say we are doing a division operation and have to get an error message when dividing by 0.

typealias ErrorMessage = String

func divide<Number>(
    number dividend: Number,
    by divisor: Number
) -> Either<ErrorMessage, Number>
where Number: FloatingPoint {
    guard !divisor.isZero else {
        return .left("'\(dividend)' cannot be divided by '\(divisor)'")
    }
    return .right(dividend / divisor)
}

// ...

let eitherValue = divide(number: <some-number>, by: <divisor>)

switch eitherValue {
case let .left(errorMessage):
    presentErrorAlert(with: errorMessage) // '<some-number>' cannot be divided by '0.0'
case let .right(result):
    updateView(with: result)
}