FunctionalApproaches.swift

// ***
// Option 1 (functional approach, minimum variables)
// ***
let pagingTuple: (String?, String?) = try zip(
     dataNode.validatedOptionalValue(forKey: "after"),
     dataNode.validatedOptionalValue(forKey: "before")
)

let pagingMarker = unwrap(pagingTuple)
    .flatMap { RedditListingResult<ServerModel>.PagingMarker(before: $0.0, after: $0.1) }


// ***
// Option 2 (classic swifty way)
// ***
var pagingMarker: RedditListingResult<ServerModel>.PagingMarker? = nil

if let after = try dataNode.validatedOptionalValue(forKey: "after") as String?,
   let before = try dataNode.validatedOptionalValue(forKey: "before") as String?
{
    pagingMarker = .init(before: before, after: after)
}


// ***
// Functional helper methods
// ***

func zip<T, P>(_ value1: @autoclosure () throws -> T, _ value2: @autoclosure () throws -> P) throws -> (T, P) {
    return (try value1(), try value2())
}

func unwrap<T, P>(_ tuple: (T?, P?)) -> (T, P)? {
    return (tuple.0 != nil && tuple.1 != nil) ? (tuple.0!, tuple.1!) : nil
}