Functional date formatting

DateFormattingPlayground.swift

import Foundation

extension Date {
    typealias FormattedString = (DateFormatter) -> String

    func formatted(by format: String) -> FormattedString {
        return { formatter in
            formatter.dateFormat = format
            return formatter.string(from: self)
        }
    }
}

extension String {
    typealias ParsedDate = (DateFormatter) -> Date?

    func parse(as format: String) -> ParsedDate {
        return { formatter in
            formatter.dateFormat = format
            return formatter.date(from: self)
        }
    }
}

let date = Date()

let formatter = DateFormatter()
formatter.locale = Locale(identifier: "bg_BG")
formatter.timeZone = TimeZone(abbreviation: "EEST")

let localised = date.formatted(by: "d MMMM, YYYY")(formatter)
print("Formatted with locale: \(localised)")

let formatted = date.formatted(by: "yyyy-MM-dd'T'HH:mm:ssZZZZZ")(formatter)
print("Formatted using RFC3339: \(formatted)")

if let outputDate = formatted.parse(as: "yyyy-MM-dd'T'HH:mm:ssZZZZZ")(formatter) {
    print("Parsed from RFC3339 formatted string: \(outputDate)")
}