This post explores how optionals help preserve strong type safety within Swift. We’re going to create a Swift version of an Objective-C API. Swift doesn’t really need this API, but it makes for a fun example.
In Objective-C, NSDictionary has a method -objectsForKeys:notFoundMarker: that takes an NSArray of keys, and returns an NSArray of corresponding values. From the documentation: “the N-th object in the returned array corresponds to the N-th key in [the input parameter] keys.” What if the third key isn’t actually in the dictionary? That’s where the notFoundMarker parameter comes in. The third element in the array will be this marker object rather than a value from the dictionary. The Foundation framework even provides a class for this case if you don’t have another to use: NSNull.
In Swift, the Dictionary type doesn’t have an objectsForKeys equivalent. For this exercise, we’re going to add one — as valuesForKeys in keeping with the common use of ‘value’ in Swift — using an extension:
extension Dictionary {
func valuesForKeys(keys: [K], notFoundMarker: V) -> [V] {
// To be implemented
}
}