Advanced and Practical Enum usage in Swift

Generic Enums

released Fri, 01 Mar 2019
Swift Version 5.0

Generic Enums

Enums can also be defined over generic parameters. You'd use them to adapt the associated values of an enum. The simplest example comes straight from the Swift standard library, namely the Optional type. You probably mostly use it with optional chaining (?), if let, guard let, or switch, but syntactically you can also use Optionals like so:

let aValue = Optional<Int>.some(5)

let noValue = Optional<Int>.none

if noValue == Optional.none { print(\"No value\") }

This is the direct usage of an Optional without any of the syntactic sugar that Swift adds in order to make your life a tremendous amount easier. If you look at the code above, you can probably guess that internally the Optional is defined as follows 1:

// Simplified implementation of Swift's Optional

enum MyOptional<T> {

   case some(T)

   case none


What's special here is, that the enum's associated values take the type of the generic parameter T, so that optionals can be built for any kind you wish to return.

Enums can have multiple generic parameters. Take the well-known Either type which is not part of Swift's standard library but implemented in many open source libraries as well as prevalent in other functional programming languages like Haskell or F#. The idea is that instead of just returning a value or no value (nÊe Optional) you'd return either one of two different values.

For example, if you parse user input, the user could enter a name or a number, in that case the type of Either would be Either<String, Int>.

enum Either<T1, T2> {

   case left(T1)

   case right(T2)


Finally, all the type constraints that work on classes and structs in Swift also work on enums. Here, we have a type Bag that is either empty or contains an array of elements. Those elements have to be Equatable.

enum Bag<T: Sequence> where T.Iterator.Element==Equatable {

     case empty

     case full(contents: [T)]