Trying to see what casting options exist with an Either type.

Either.swift

public enum Either<Left, Right> {
    case left(Left)
    case right(Right)
}

extension Either {

    public func `as`(_ type: Left.Type) -> Left? {
        guard case let .left(left) = self else { return nil }
        return left
    }

    public func `as`(_ type: Right.Type) -> Right? {
        guard case let .right(right) = self else { return nil }
        return right
    }

    public func `as`<L,R>(_ type: L.Type) -> L? where Left == Either<L,R> {
        guard case let .left(left) = self else { return nil }
        return left.as(type)
    }

    public func `as`<L,R>(_ type: R.Type) -> R? where Left == Either<L,R> {
        guard case let .left(left) = self else { return nil }
        return left.as(type)
    }

    public func `as`<L,R>(_ type: L.Type) -> L? where Right == Either<L,R> {
        guard case let .right(right) = self else { return nil }
        return right.as(type)
    }

    public func `as`<L,R>(_ type: R.Type) -> R? where Right == Either<L,R> {
        guard case let .right(right) = self else { return nil }
        return right.as(type)
    }
}

EitherUse.swift

let either = Either<Either<String, Int>, Either<Float, Double>>.left(.left("Hello"))
print(either.as(String.self))
print(either.as(Int.self))
print(either.as(Float.self))
print(either.as(Double.self))

let either2 = Either<Either<Either<String, Int>, Double>, Float>.left(.left(.left("Hello")))
print(either2.as(String.self)) // Fails to compile (obviously)
print(either2.as(Int.self)) // Fails to compile (obviously)
print(either2.as(Float.self))
print(either2.as(Double.self))