folding_result.swift

func canFail() -> Result<Int, Error> {
  Bool.random()
    ? .success(7)
    : .failure(NSError())
}

func neverFail() -> Result<Int, Never> { .success(9) }

func absurd<A>(_ never: Never) -> A {}

extension Result {
  func fold<T>(ifSuccess: (Success) -> T, ifFailure: (Failure) -> T) -> T {
    switch self {
    case let .success(a): return ifSuccess(a)
    case let .failure(e): return ifFailure(e)
    }
  }
}

let label1 = UILabel()
label1.text = canFail().fold(
  ifSuccess: { "Received: \($0)" },
  ifFailure: { "Fetch failed: \($0.localizedDescription)" }
)

let label2 = UILabel()
label2.text = neverFail().fold(
  ifSuccess: { "Received: \($0)" },
  ifFailure: absurd
)