A neat way to avoid nil checks with collections...

OptionalChecking.swift

let array = ["a", "b", "c", "d"].map(Optional.init)
array.contains("a") // true
array.contains(Optional("a")) // true
array.contains("e") // false
array.contains(Optional("e")) // false

let otherArray = ["a"]

// `Array.first` is an Optional property, but we can use it as an argument to `Array.contains` since the array is actually of Optional<String> type...
array.contains(otherArray.first) // true