Nil-rejection operator example

NilRejectionExample.swift

infix operator !! : NilCoalescingPrecedence

func !!<UnwrappedType: Any, ErrorType: Error>(lhs: Optional<UnwrappedType>, rhs: ErrorType) throws -> UnwrappedType {
    guard let unwrappedValue = lhs else {
        throw rhs
    }
    return unwrappedValue
}

/****************************************************************************************************/
/* Dictionary access example                                                                        */
/****************************************************************************************************/

enum Errors: Error {
    case keyNotFound(key: String)
    case oneOfKeysNotFound(keys: [String])
}

let sampleDictionary: [String: Int] = [
    "First":    0,
    "Second":   1,
    "Fourth":   2,
    "Fifth":    3
]

// Nil-rejection on a single item
do {
    let first = try sampleDictionary["First"] !! Errors.keyNotFound(key: "First")
    let second = try sampleDictionary["Second"] !! Errors.keyNotFound(key: "Second")
    let third = try sampleDictionary["Third"] !! Errors.keyNotFound(key: "Third")
    let fourth = try sampleDictionary["Fourth"] !! Errors.keyNotFound(key: "Fourth")
} catch let e {
    print(e)
}

// Nil-rejection with nil-coalescence
do {
    let first =
        try sampleDictionary["First"] ??
            sampleDictionary["Second"] !!
            Errors.oneOfKeysNotFound(keys: ["First", "Second"])
    
    try sampleDictionary["Third"] ??
        sampleDictionary["Second"] !!
        Errors.oneOfKeysNotFound(keys: ["Third", "Second"])
    
    try sampleDictionary["Third"] ??
        sampleDictionary["Sixth"] !!
        Errors.oneOfKeysNotFound(keys: ["Third", "Sixth"])
    
    try sampleDictionary["Third"] ??
        sampleDictionary["Sixth"] ??
        sampleDictionary["Seventh"] !!
        Errors.oneOfKeysNotFound(keys: ["Third", "Sixth", "Seventh"])
    
    

} catch let e {
    print(e)
}