optional_arguments_application.swift

infix operator .?  : NilCoalescingPrecedence

func .?<ARG, RES> (fn: (ARG) -> RES, arg: ARG?) -> RES? {
    if let arg = arg {
        return fn(arg)
    }
    return nil
}

func .?<ARG0, ARG1, RES> (fn: (ARG0, ARG1) -> RES, arg: (ARG0?, ARG1?)) -> RES? {
    if let arg0 = arg.0, let arg1 = arg.1 {
        return fn(arg0, arg1)
    }
    return nil
}

// etc


let add: (Int,Int) -> Int = {$0 + $1}
let inc = {$0 + 1}

add .? (1, 2)                       // 3
add .? (nil, 2)                     // nil


// check precedence

inc .? nil ?? 1 + 1                 // 3
(inc .? nil ?? 1 + 1) ?? 0 > 2      // true