optionaIsEqual.swift

extension Optional {
    func both_then<T, U>(_ other: Optional<U>, f: (Wrapped, U) -> T) -> Optional<T> {
        guard let a = self, let b = other else { return nil }

        return f(a, b)
    }

    func neither<T>(_ other: Optional<T>) -> Bool {
        return self == nil && other == nil
    }
}

extension Optional where Wrapped: Equatable {
    func isEqual(_ other: Optional<Wrapped>) -> Bool {
        return self.neither(other) || self.both_then(other, f: ==) ?? false
    }
}

let a: Int? = nil
let b: Int? = nil
let c: Int? = 2
let d: Int? = 2
let e: Int? = 3

assert(a.isEqual(b))
assert(c.isEqual(d))
assert(a.isEqual(c) == false)
assert(d.isEqual(e) == false)