Language Design: Nondefinitional Enums

Published on 2021-08-26. Last updated on 2022-05-27


The following piece of code shows a “classic” interpretation of an enum (ADT) definition as it exists in many languages …

enum Pet { Cat(name: String, lives: Int), Dog(name: String, age: Int) }

… in which Pet (the enum itself), as well as Cat and Dog (enum members of Pet), are defined.

Similarly, many languages have unions types, often written as …

type Number = Int | Float

in which the union type Number is defined, with its members Int and Float refering to existing types.

We observe that ADTs are generally tagged unions (meaning that individual members can be told apart, even if they contained the same values) come with wrappers (Cat, Dog) around their payloads, while untagged unions do not contain metadata to tell members apart, but also lack wrappers.

Syntactic Wrapping No Syntactic Wrapping
Runtime Tagging tagged union/ADT/enum ?
No Runtime Tagging ? untagged union/union type

How to fill the gaps?

Let’s call the tagged union without syntactic wrapping combination in the upper right quadrant a nondefinitional enum. Consider this code …

enum Pet of Cat, Dog
class Cat(name: String, lives: Int)
class Dog(name: String, age: Int)

… that defines the enum Pet, but refers to existing types Cat and Dog, instead of defining them too.

Intuitively, this works similarly to permits clauses of sealed interfaces in Java in the sense that

sealed interface E permits A, B { ... }

does not define A or B, but refers to existing A and B in scope.1

Benefits of nondefinitional enums

  1. Enum variants have types, because they have a “real” class/struct/… declaration. (This fixes a mistake that some languages like Rust or Haskell made.)
  2. Variants can be reference types or value types (because they have a “real” class/struct/… declaration).
  3. No “stutter”, where variant names have to be invented to wrap existing types- (Rust has this issue.)
  4. enum values can be passed/created more easily, because there are fewer layers of wrapping.
  5. Variants can be re-used in different enums.
  6. The ability to build ad-hoc enums out of existing types obviates the need for a separate union type or type alias feature in the language.

Example for 1., 2., 3.

So while

enum Option[T] { Some(value: T), None }

would receive little benefit from being written as …

enum Option[T] of Some[T], None
struct Some[T](value: T)
module None

… even trivial ADTs like a JSON tree would benefit. Instead of …

enum JsonValue {
  JsonObject(Map[String, JsonValue])
  JsonArray (Array[JsonValue]),
  JsonBool  (Bool),

… one would write (with Array, Float64 and String being existing types in the language):

enum JsonValue of
  Map[String, JsonValue]

module JsonNull

Example for 4.

It would also do away with having to wrap data the enum’s “variant” when passing arguments, as it’s done with the “traditional” approach:

fun someValue(value: JsonValue) = ...
someValue(JsonString("test")) // "traditional" approach
someValue("test")             // could also be allowed

Example for 5.

Consider a class like

class Name(name: String)

With this approach we can use this Name type multiple times in different enums (and elsewhere):

enum PersonIdentifier of
  ... // other identifiers like TaxId, Description, PhoneNumber etc.

enum DogTag of
  ... // other identifiers like RegId, ...

This approach reduces indirection at use-sites and increases the utility of enums compared to more “traditional” enums, while not changing their runtime costs or representation.

  1. Unlike sealed interfaces in Java though, E and A/B have no subtyping relationship in nondefinitional enums.