As of mid-2020, there is some discussion of adding enumerations (enums) to PHP. There are many good reasons to do so, most around enabling better data modeling and type checking, but that doesn't suggest how to do it. Enumerations in practice refer to a very wide range of functionality depending on the language, from barely above constants to a core part of the type system.
As I am wont to do, I decided the best thing to do would be to survey the existing marketplace and see what other languages did, and what we can steal outright. (As the saying goes, "PHP evolves by beating up other languages in dark alleys and going through their pockets for loose syntax.") I therefore looked at 12 different languages with some kind of native enumeration support. The survey below is intended as a reasonably fair overview and summary of the available languages. My own thoughts and analysis are included at the end. For some languages I have included runnable sample code in the appropriate subdirectory. Whether or not there is sample code depends primarily on whether I had a runtime for the language already installed.
I deliberately excluded languages with no native enum support. Languages such as Javascript, Go, or Ruby do not (as far as I can tell) have any native enumerations, although there are various hacky ways to simulate them in user space. That is not of interest to us at this time.
If you spot any errors in the survey below, please let me know.
In C, enumerations are really just a wrapper for named integer constants. They are defined with the keyword enum, usually in combination with typedef. For example:
#include<stdio.h>
typedef enum { Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday } Day;
void printer(Day d) {
printf("The day is: %d\n", d);
}
int main(void) {
Day d = Tuesday;
printer(d);
printer(4);
return 0;
}Even though printer() takes a Day parameter, passing an integer literal works fine. If no integer value for an enum element is specified, the compiler assigns one automatically starting from 0. So Monday is 0, Tuesday is 1, etc. You can specify an equivalent integer for an enum value, including making multiple values refer to the same integer:
typedef enum {
Working,
Failed = 5,
Busted = 5;
} Status;Note that even though d is of Day type, the enum constant Tuesday is defined in the global scope. That is, the following code does not compile, as Monday is defined twice.
typedef enum { Tuesday = 1, Monday, Wednesday } WeirdDays;
typedef enum { Monday, Tuesday, Wednesday } Day;C++ is backwards-compatible with C, so the previous section applies. In addition, starting with C++11, scoped enumerations (defined with enum struct or enum class) have been introduced.
The enums are defined the same way as in C (so individual enumerators' values can be specified, etc.). There is no automatic conversion from the scoped enum type to the underlying integer type.
Note: Even though the defining keywords are enum struct, the type itself does not behave like a struct: no fields or member methods can be defined.
#include <iostream>
typedef enum { Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday } Day;
enum struct ScopedDay { Monday = 9, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday };
void printer(Day d) {
std::cout << "The classical day is " << d << '\n';
}
void printer(ScopedDay d) {
std::cout << "The scoped day is " << static_cast<int>(d) << '\n';
}
int main() {
Day d1 = Tuesday;
ScopedDay d2 = ScopedDay::Tuesday;
printer(d1); // 1
printer(d2); // 10
return 0;
}In Java, enums are, unsurprisingly, a shorthand for classes with class constants. They can be defined standalone or within a class, since Java supports inner classes. As a result, enums can support arbitrary methods. The specific values can map to internal integer values, or they can be auto-assigned by the compiler.
The simple case looks like this:
enum Suit {
HEARTS,
DIAMONDS,
CLUBS,
SPADES
}Enum values have a number of methods on them by default to access metadata, including Suit.valueOf("HEARTS") (returns "HEARTS") and Suit.valueOf("HEARTS").ordinal() (returns 0).
The values of an enum can be iterated as a set:
for (Suit s : Suit.values()){
System.out.println(s);
} Because they're built on classes, enums can have methods.
enum Suit {
HEARTS,
DIAMONDS,
CLUBS,
SPADES;
public String color() {
switch (this) {
case SPADES:
return "Swords of a soldier";
case CLUBS:
return "Weapons of war";
case DIAMONDS:
return "Money for this art";
default:
return "Shape of my heart";
}
}
}The switch statement is not exhaustive on enums, however.
Enum constructors are always private, so they can only be called from the definition of enum members. They also support interfaces.
enum Suit {
HEARTS("H"),
DIAMONDS("D"),
CLUBS("C"),
SPADES("S");
private String abbrev;
Suit(String abbrev) {
this.abbrev = abbrev;
}
String shortName() {
return abbrev;
}
}Further reading: https://www.javatpoint.com/enum-in-java
Python builds its enum support on top of classes. An "enum class" is simply a class that extends the enum.Enum parent, which has a lot of methods pre-implemented to provide Enum-ish behavior. All properties of the class are enum members:
import enum
class Suit(enum.Enum):
HEARTS = enum.auto()
DIAMONDS = enum.auto()
CLUBS = 'C'
SPADES = "S"Enum members can be any int or string primitive, or can be auto-generated. The auto-generation logic can also be overridden by defining a _generate_next_value_() method in the class. When an enum value is cast to a string, it always shows as Card.CLUBS or similar, but can be overridden by implementing the __str__ method.
Enum member names must be unique, but values need not be. If two members have the same value then the syntactically first one wins, and all others are alises to it. The aliases will be skipped when iterating an enum or casting it to a list. If needed, you can get the original list with Card.__members__.items().
As a class, an enum can also have methods. However, the methods have no native way to vary depending on which enum value they're on. You can check the value within the method, though:
class Suit(enum.Enum):
HEARTS = enum.auto()
DIAMONDS = enum.auto()
CLUBS = 'C'
SPADES = "S"
def color(self):
if self in [self.CLUBS, self.SPADES]:
return "Black"
else:
return "Red"Because Python lacks any meaningful type declarations on variables, parameters, or return values, there's no way to restrict a value to an enum list. Enum classes also cannot be extended.
The Enum class also has an alternate function-style syntax for simple cases:
Suit = Enum('Suit', 'HEARTS DIAMONDS CLUBS SPADES')Further reading: https://docs.python.org/3/library/enum.html
Typescript supports primitive enumerations, including both constant and runtime-defined values. Depending on the details they may or may not get compiled away to literal constants in code. It has its own dedicated keyword.
enum Suit {
Hearts,
Diamonds,
Clubs,
Spades,
}is equivalent to
enum Suit {
Hearts = 0,
Diamonds = 1,
Clubs = 2,
Spades = 3,
}Enums can also have string values if specified explicitly. Values can be set based on some other value, even function definitions:
enum FileAccess {
// constant members
None,
Read = 1 << 1,
Write = 1 << 2,
ReadWrite = Read | Write,
// computed members
UserSetting = userDefaultValue()
}Normally enums exist at runtime, but a fully-constant enum can also be flagged to compile-away to raw constants in the source code:
const enum ShouldWe {
No,
Yes,
}Enum types can be used as type declarations:
function pickCard(desiredSuit: Suit): Card { }Further reading: https://www.typescriptlang.org/docs/handbook/enums.html
Strictly speaking Haskell doesn't have enums, but the way its type system works gives you something close enough that I'm going to include it. In Haskell, you define a new data type with the data keyword, which can be defined in terms of other data types and type constructors.
It's really hard to explain without going into the whole type system, so I'll stick to some examples:
data Suit = Hearts | Diamonds | Clubs | SpadesThe type "Suit" has only four values, one for each suit. They are not backed by a primitive value but literally are those values only. Haskell doesn't have methods as we'd understand them in the OOP world, and I've not been able to wrap my brain around Haskell enough to say if you can attach methods consistently to types of an Enum. They can, however, be used in pattern matching:
data Color = Red | Black
suitColor :: Suit -> Color
suitColor Hearts = Red
suitColor Diamonds = Red
suitColor _ = BlackBecause type values are technically not values but "type constructors" they can be parameterized by other values. For instance, the infamous Maybe Monad is defined as:
data Maybe a = Just a | NothingThat is, a "Maybe" can be either the literal Nothing or a Just combined with some other value, which can then be extracted later using pattern matching.
stuff :: Maybe a -> Int
stuff Nothing = 0
stuff Just a = aFurther reading: https://wiki.haskell.org/Type
F#, in what seems to be a very on-brand move, has both union types and enums. They are very similar but not quite the same thing.
Union types in F# look and act an awful lot like Haskell, including the requirement that the unioned types start with a capital.
type SuitUnion = Hearts | Diamonds | Clubs | SpadesThey have no underlying primitive equivalent. F#'s match directive forces you to enumerate all possible values, to help avoid errors:
let color = match x with
| Hearts -> Red
| Diamonds -> Red
| Clubs -> Black
| Spades -> BlackEnums in F#, by contrast, are backed by underlying integer primitives that you specify. Strings are not allowed. They can be all lowercase if you want, but have to be qualified when referencing to them:
type SuitEnum = Hearts = 1 | diamonds = 2 | Clubs = 3 | Spades = 4
let color = match x with
| SuitEnum.Hearts -> "Red"
| SuitEnum.diamonds -> "Red"
| SuitEnum.Clubs -> "Black"
| SuitEnum.Spades -> "Black"
| _ -> "What kind of deck are you using?"Enums can be cast to and from integers. That also, oddly, allow you to define an enum value that is out of range.
// This is, amazingly, legal.
let horseshoe = enum<SuitEnum>(5)For that reason, the _ fallback match arm is required for enums, but not for unions.
Because F# doesn't have function parameter or return types, neither unions nor enums can be type defined in a function signature.
Further reading: https://fsharpforfunandprofit.com/posts/enum-types/
C# enums are explicitly just named integer constants, much like in C. They can be defined within a class like constants, or (I think) stand-alone with a namespace.
enum Suits
{
Hearts = 0,
Diamonds,
Clubs,
Spades
}If a value is not specified, it will be set to the highest existing value + 1. 0 is the default first value but you can set your own. They are referenced scoped, so Suits.Diamonds, Suits.Spades, etc.
Values can also be defined based on other enum values, bitmask style, such as RedCards = Hearts|Diamonds. However, that only works if the explicit values are defined as bit flags.
Enums need to be cast to an integer explicitly in order to use as an int.
Console.WriteLine((int)WeekDays.Monday);An Enum class contains various static methods for manipulating enumerations further. For instance, to get a list of the names in a given enumeration:
foreach (string str in Enum.GetNames(typeof(WeekDays))) {
Console.WriteLine(str);
}Or this somewhat crazy way to cast an integer up to an enum member:
WeekDays wdEnum;
Enum.TryParse<WeekDays>("1", out wdEnum);
Console.WriteLine(wdEnum);Although they're not a class, you can technically add "extension methods" to enums that end up looking kind of like them. For instance:
public static string Color(this Suit s) {
switch (s)
{
case Hearts: return "Red";
case Diamonds: return "Red";
case Clubs: return "Black";
case Spades: return "Black";
}
}
var theColor = Suit.Clubs.Color();Further reading: https://www.tutorialsteacher.com/csharp/csharp-enum
Swift's enumerations are closer to union types, but still called enumerations. (Go figure.) They form a full fledged type with limited legal values. That means the type has to be capitalized, and the values not.
enum Suit {
case hearts
case diamonds
case clubs
case spades
}
// or
enum Suit {
case hearts, diamonds, clubs, spades
}Once defined, values can be defined of that type, and Swift's type inference capability can shorten the syntax somewhat.
var card = Suit.clubs
// since card is now bound to the type Suit, you can now do this:
card = .spadesYou can match on an enum value with switch, and it must either be exhaustive or have a default:
switch card {
case .spades:
print("The swords of a soldier.")
case .clubs:
print("Weapons of war.")
case .diamonds:
print("Money for this art.")
default:
print("That's not the shape of my heart.")
}Enums are not natively iterable, but they can be converted into that easily:
enum Suit: CaseIterable {
case hearts, diamonds, clubs, spades
}
for s in Suit.allCases {
print(s)
}Swift allows enums to have what it calls "associated values," creating what is variously called a "discriminated union" or "tagged union" depending on whom you ask. Each value can have its own set of associated values that could be the same or different.
case Suit {
case hearts(String)
case diamonds(String)
case clubs(String)
case spades(String)
}
var threeOfDiamonds = Suit.diamond("3")Each instance of an associated value enum is then not equal to another, even if they're of the same enum value. Seemingly the only way to get those values back out, though, is with pattern matching:
switch card {
case .spades(let value):
print("The \(value) of Spades")
case .clubs(let value):
print("The \(value) of Clubs")
case let .diamonds(value):
print("The \(value) of Diamonds")
case let .hearts(value):
print("The \(value) of Hearts")
}For one-off cases, you can use if let.
if case let .clubs(val) = card {
print ("The \(val) of Clubs")
}(Those all do the same thing, but digging into the intricacies of Swift's pattern matching is out of scope for now.)
Enums can also support "raw values," if specified explicitly, but they must be of the same primitive type:
enum Suit: Character {
case hearts = "H"
case diamonds = "D"
case clubs = "C"
case spades = "S"
}If you list only one raw value, Swift will try to generate a raw value for the rest based on the type used. It's also possible to initialize an enum case from a raw value, if one was defined:
let card = Suit(rawValue: "B")This actually creates an "optional" of type Suit?, meaning it may or may not be legal and you have to explicitly check it. (Optionals are essentially a syntactic Maybe Monad, and way off topic.)
You can even define an enumeration in terms of itself, which is just all kinds of weird. From the documentation:
indirect enum ArithmeticExpression {
case number(Int)
case addition(ArithmeticExpression, ArithmeticExpression)
case multiplication(ArithmeticExpression, ArithmeticExpression)
}And they go further by supporting methods on enumerations of all of the above types, the body of which would meaningfully have to be a switch:
case Suit {
case hearts(String)
case diamonds(String)
case clubs(String)
case spades(String)
func color: String {
switch self {
case .hearts: return "Red"
case .diamonds: return "Red"
case .clubs: return "Black"
case .spades: return "Black"
}
}
}
print(Suit.clubs("3").color());
// Prints "Black"Further reading: https://docs.swift.org/swift-book/LanguageGuide/Enumerations.html
As Rust's main syntactic goal seems to have been "Haskell, but with lots of curly braces," the language supports enumerations with and without associated values, either positional or named.
All of the following are legal:
// The values themselves.
enum Suit {
Hearts,
Diamonds,
Clubs,
Spades,
}
// With one or more tuple associated values.
enum Card {
Hearts(i8),
Diamonds(i8),
Clubs(i8),
Spades(i8),
}
// With one or more struct-associated values.
enum Card {
Hearts{val: i8},
Diamonds{val: i8},
Clubs{val: i8},
Spades{val: i8},
}
// With an integer (only) explicit value.
enum Suit {
Hearts = 3,
Diamonds = 4,
Clubs = 5,
Spades = 6,
}Enum values can be referenced scoped from their type, Suit::Heart, or first imported with use Suit::* and then used unqualified. Because they're a full type, they can be used in function signatures.
Enums are almost always used with either match or if let, the latter of which being a sort of inverted way to care about only a single branch of a match. The match version must be exhaustive or have a default.
let msg = match card {
Suit::Spades => "Swords of a soldier".to_string(),
Suit::Clubs => "Weapons of war".to_string(),
Suit::Diamonds => "Money for this art".to_string(),
_ => "Shape of my heart".to_string(),
};
if let Diamonds(val) = card {
println!("{} diamonds are a girl's best friend", val)
}The only way to extract associated values out of the enum is with pattern matching, which in Rust is almost absurdly robust:
use Card::*;
let the_val = match Card {
Clubs(x) | Hearts(x) | Spades(x) | Diamonds(x) => x
};As it's a full type, it can also have methods. Or in Rust-speak, "implementations," including of traits (what most languages would call an interface). They can do pretty much everything a struct can. Their body will in most cases be just a bit match.
impl Suit {
fn color(&self) -> String {
match self {
Self::Hearts => "Red".to_string(),
Self::Diamonds => "Red".to_string(),
Self::Clubs => "Black".to_string(),
Self::Spades => "Black".to_string(),
}
}
}(The capitalized Self in this case is an implicit alias to Suit.)
Further reading: https://doc.rust-lang.org/rust-by-example/custom_types/enum.html
Kotlin also has not one but two enum-esque systems: Enums and Sealed Classes. The difference between them is subtle and confusing.
Enums are a class that inherits from an Enum class implicitly.
enum class Suit {
HEARTS,
DIAMONDS,
CLUBS,
SPADES
}Each enum value is technically a "constant object." By default they're bare, but can also take int or string values constructor-style.
enum class Suit(val abbrev: String) {
HEARTS("H"),
DIAMONDS("D"),
CLUBS("C"),
SPADES("S")
}Enums in Kotlin support methods, and unlike most languages here the methods may be defined separately for each value. Technically they're all implemented as subclasses, with the parent as an abstract base class. An enum can even support interfaces.
interface Colorable {
fun color()
}
enum class Suit(val abbrev: String) {
HEARTS("H") {
override fun color(): String = "Red"
},
DIAMONDS("D") {
override fun color(): String = "Red"
},
CLUBS("C") {
override fun color(): String = "Black"
},
SPADES("S") {
override fun color(): String = "Black"
};
abstract fun color(): String
fun abbreviation(): String {
return this.abbrev
}
}Enums have a number of built-in methods and properties, which make it possible to iterate an enum or get its value. That also makes them trivially serializable, unlike Sealed Classes.
Sealed Classes, meanwhile, are almost like normal classes except that the list of subclasses is fixed at compile time and they must appear in the same source file.
Whereas Enums are singletons, sealed classes may be singleton or instance-based.
sealed class Action
// This is a singleton sealed class
object Quit: Action()
// This is an instance-able sealed class
class Move(val dir: String): Action() Since they're objects/methods in their own right, they can have whatever methods you want, inherited or not. However, they do not have the automatic methods or properties of Enums that make them serializable.
Kotlin supports a when syntax as an alternative to switch that is an expression, and can, in some cases, detect exhaustiveness.
var result = when (card) {
Suit.SPADES -> "The swords of a soldier"
Suit.CLUBS -> "Weapons of war"
Suit.DIAMONDS -> "Money for this art"
else -> "The shape of my heart"
}Further reading: https://blog.kotlin-academy.com/enum-vs-sealed-class-which-one-to-choose-dc92ce7a4df5
Scala enums are also built on objects.
package com.crell.poker {
object Suit extends Enumeration {
type Suit = Value
val HEARTS, DIAMONDS, CLUBS, SPADES = Value
}
}
// ...
object Main extends App {
import com.crell.poker.Suit._
var s = CLUBS
// Iteration
Suit.values foreach println
}They can carry values, including multiple values, which must be pre-set and not vary by instance. They also can support methods that way, although my Scala-fu is not strong enough to know if my syntax here is entirely correct. :-)
object Suit extends Enumeration {
protected case class Val(abbrev: String) extends super.Val {
def color: String = abbrev.match {
case Suit.HEARTS => "Red"
case Suit.DIAMONDS => "Red"
case Suit.CLUBS => "Black"
case Suit.SPADES => "Black"
}
}
type Suit = Value
val HEARTS = Val("H")
val DIAMONDS = Val("D")
val CLUBS = Val("C")
val SPADES = Val("S")
}Further reading: https://www.scala-lang.org/api/current/scala/Enumeration.html
Folded into a convenient table, a feature summary looks like this:
| Language | C/C++ | Java | Python | Typescript | Haskell | F# (Union) | F# (Enum) | C# | Swift | Rust | Kotlin (Enums) | Kotlin (Sealed) | Scala |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Unit values | No | No | No | No | Yes | Yes | No | No | Yes | Yes | Yes | Ish? | Yes |
| Int values | Yes | Yes | Yes | Yes | No | No | Yes | Yes | Yes | Yes | Yes | Ish? | Yes |
| String values | No | Yes | Yes | Yes | No | No | No | No | Yes | No | Yes | Ish? | Yes |
| Associated values | No | No | No | No | Yes | No | No | No | Yes | Yes | No | Yes | No |
| Methods | No | Yes | Yes | No | No? | No | No | Ish | Yes | Yes | Yes | Yes | Yes |
| Type checked | Ish | Yes | No | Yes | Yes | No | Ish | Yes | Yes | Yes | Yes | Yes | Yes? |
| Iterable | No | Yes | Yes | No | No | No | No | Yes | Yes | No | Yes | No | No |
In terms of overall capability, Swift appears to have the edge with Rust a very close second. However, Rust also seems to have more powerful associated values ability (tuples or structs), and the usefulness of iterating enum types is debatable. I'm going to call it a qualified tie between those two in raw expressive power.
Broadly speaking, I would separate the languages into a few categories:
While they are superficially similar, and often use the same terminology, they approach the problem from different ways. The Fancy Constants languages are offering a syntactic convenience, but little else. Often they get compiled away at runtime, and their type checking may be incomplete.
The Fancy Objects languages take that a step further and offer methods on enum types, which offers a centralized place to put a switch, match, or whatever branching syntax for RTTI. That is helpful, and helps with data modeling in ways that Fancy Constants do not. If the methods need to vary by enum type more than just a little, though, you run into some contortions and may find yourself better off with normal objects and interfaces.
The main differentiator for ADT languages, as I'm using them here, is that they can be parameterized with different values. That offers another layer again of potential functionality and data modeling. It also becomes a natural and easy way to implement Monads in user space, and Haskell, Swift, and Rust all do exactly that in their core libraries, particularly for Maybe/Optional. That makes them an extremely robust way to handle data modeling in your application, and to "make invalid states unrepresentable," which is an excellent feature if you can get it.
The downside is that once you start parameterizing enum values, you no longer get a guarantee that a Club is a Club is a Club. They may well be two different Clubs. The implementation details here around equality (a tricky subject in the best of circumstances) are the devil's hiding place. The other catch is that, as far as I can tell, no language with parameterized enum values lets you get at them easily without doing pattern matching. Depending on your use case that may be no big deal or may be a deal-breaker. In practice, I think it largely comes down to how easy the syntax is for pattern matching; Of all things I'd say Haskell is the nicest here, followed by Swift, then Rust. (Or possibly Rust then Swift, depending on your tastes. Rust gets very tricky when you have struct-parameterized enums.)