Backed enumerations

By default, Enumerated Cases have no scalar equivalent. They are simply singleton objects. However, there are ample cases where an Enumerated Case needs to be able to round-trip to a database or similar datastore, so having a built-in scalar (and thus trivially serializable) equivalent defined intrinsically is useful.

To define a scalar equivalent for an Enumeration, the syntax is as follows:

<?phpenum Suit: string{    case Hearts = 'H';    case Diamonds = 'D';    case Clubs = 'C';    case Spades = 'S';}?>

A case that has a scalar equivalent is called a Backed Case, as it is "Backed" by a simpler value. An Enum that contains all Backed Cases is called a "Backed Enum." A Backed Enum may contain only Backed Cases. A Pure Enum may contain only Pure Cases.

A Backed Enum may be backed by types of int or string, and a given enumeration supports only a single type at a time (that is, no union of int|string). If an enumeration is marked as having a scalar equivalent, then all cases must have a unique scalar equivalent defined explicitly. There are no auto-generated scalar equivalents (e.g., sequential integers). Backed cases must be unique; two backed enum cases may not have the same scalar equivalent. However, a constant may refer to a case, effectively creating an alias. See Enumeration constants.

Equivalent values must be literals or literal expressions. Constants and constant expressions are not supported. That is, 1 + 1 is allowed, but 1 + SOME_CONST is not.

Backed Cases have an additional read-only property, value, which is the value specified in the definition.

<?phpprint Suit::Clubs->value;// Prints "C"?>

In order to enforce the value property as read-only, a variable cannot be assigned as a reference to it. That is, the following throws an error:

<?php$suit = Suit::Clubs;$ref = &$suit->value;// Error: Cannot acquire reference to property Suit::$value?>

Backed enums implement an internal BackedEnum interface, which exposes two additional methods:

The from() and tryFrom() methods follow standard weak/strong typing rules. In weak typing mode, passing an integer or string is acceptable and the system will coerce the value accordingly. Passing a float will also work and be coerced. In strict typing mode, passing an integer to from() on a string-backed enum (or vice versa) will result in a TypeError, as will a float in all circumstances. All other parameter types will throw a TypeError in both modes.

<?php$record = get_stuff_from_database($id);print $record['suit'];$suit =  Suit::from($record['suit']);// Invalid data throws a ValueError: "X" is not a valid scalar value for enum "Suit"print $suit->value;$suit = Suit::tryFrom('A') ?? Suit::Spades;// Invalid data returns null, so Suit::Spades is used instead.print $suit->value;?>

Manually defining a from() or tryFrom() method on a Backed Enum will result in a fatal error.