fmap in Swift 3 to handle nil values

fmap.swift

// fmap
precedencegroup ComparisonPrecedence {
    associativity: left
}

infix operator <^> : ComparisonPrecedence

func <^><A, B>(f: (A) -> B, a: A?) -> B? {
    switch a {
    case .some(let x):
        return f(x)
    case .none:
        return .none
    }
}

// example
var nameString: String? = "Bob" // could also be nil
//nameString = nil

func usernameFormat(name: String) -> String {
    return "@\(name)"
}

let handle = usernameFormat <^> nameString
print(handle ?? "")

Inspired by https://robots.thoughtbot.com/functional-swift-for-dealing-with-optional-values