enumerations.xml

<?xml version="1.0" encoding="utf-8"?>
 <chapter xml:id="language.enumerations" xmlns="http://docbook.org/ns/docbook">
  <title>Enumerations</title>
  <sect1 xml:id="language.enumerations.overview">
   <title>Enumerations overview</title>
   <?phpdoc print-version-for="enumerations"?>

   <para>
    Enumerations, or "Enums", allow a developer to define a custom type that is limited to one
    of a discrete number of possible values. That can be especially helpful when defining a
    domain model, as it enables "making invalid states unrepresentable."
   </para>

   <para>
    Enums appear in many languages with a variety of different features. In PHP,
    Enums are a special kind of object. The Enum itself is a class, and its possible
    cases are all single-instance objects of that class. That means Enum cases are
    valid objects and may be used anywhere an object may be used, including type checks.
   </para>

   <para>
    The most popular example of enumerations is the built-in boolean type, which is an
    enumerated type with legal values &true; and &false;.
    Enums allows developers to define their own arbitrarily robust enumerations.
   </para>
  </sect1>
  <sect1 xml:id="language.enumerations.basics">
   <title>Basic enumerations</title>

   <para>
    Enums are similar to classes, and share the same namespaces as classes, interfaces, and traits.
    They are also autoloadable the same way. An Enum defines a new type, which has a fixed, limited
    number of possible legal values.
   </para>


   <programlisting role="php">
<![CDATA[
<?php

enum Suit
{
    case Hearts;
    case Diamonds;
    case Clubs;
    case Spades;
}
?>
]]>
   </programlisting>

   <para>
    This declaration creates a new enumerated type named <literal>Suit</literal>, which has
    four and only four legal values: <literal>Suit::Hearts</literal>, <literal>Suit::Diamonds</literal>,
    <literal>Suit::Clubs</literal>, and <literal>Suit::Spades</literal>. Variables may be assigned
    to one of those legal values. A function may be type checked against an enumerated type,
    in which case only values of that type may be passed.
   </para>

   <programlisting role="php">
<![CDATA[
<?php

function pick_a_card(Suit $suit)
{
    /* ... */
}

$val = Suit::Diamonds;

// OK
pick_a_card($val);

// OK
pick_a_card(Suit::Clubs);

// TypeError: pick_a_card(): Argument #1 ($suit) must be of type Suit, string given
pick_a_card('Spades');
?>
]]>
   </programlisting>

   <para>
    An Enumeration may have zero or more <literal>case</literal> definitions, with no maximum.
    A zero-case enum is syntactically valid, if rather useless.
   </para>

   <para>
    For Enumeration cases, the same syntax rules apply as to any label in PHP, see
    <link linkend="language.constants">Constants</link>.
   </para>

   <para>
    By default, cases are not intrinsically backed by a scalar value. That is, <literal>Suit::Hearts</literal>
    is not equal to <literal>"0"</literal>. Instead, each case is backed by a singleton object of that name. That means that:
   </para>

   <programlisting role="php">
<![CDATA[
<?php

$a = Suit::Spades;
$b = Suit::Spades;

$a === $b; // true

$a instanceof Suit;  // true
?>
]]>
   </programlisting>

   <para>
    It also means that enum values are never <literal>&lt;</literal> or <literal>&gt;</literal> each other,
    since those comparisons are not meaningful on objects. Those comparisons will always return
    &false; when working with enum values.
   </para>

   <para>
    This type of case, with no related data, is called a "Pure Case."  An Enum that contains
    only Pure Cases is called a Pure Enum.
   </para>

   <para>
    All Pure Cases are implemented as instances of their enum type. The enum type is represented internally as a class.
   </para>

   <para>
    All Cases have a read-only property, <literal>name</literal>, that is the case-sensitive name
    of the case itself.
   </para>

   <programlisting role="php">
<![CDATA[
<?php

print Suit::Spades->name;
// prints "Spades"
?>
]]>
   </programlisting>

   <para>
    It is also possible to use the <function>defined</function> and <function>constant</function>
    functions to check for the existence of or read an enum case if the name is obtained dynamically.
    This is, however, discouraged as using <link linkend="language.enumerations.backed">Backed enums</link>
    should work for most use cases.
   </para>

  </sect1>

 <sect1 xml:id="language.enumerations.backed">
  <title>Backed enumerations</title>

  <para>
   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.
  </para>

  <para>To define a scalar equivalent for an Enumeration, the syntax is as follows:</para>

  <programlisting role="php">
<![CDATA[
<?php

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

  <para>
   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.
  </para>

  <para>
   A Backed Enum may be backed by types of <literal>int</literal> or <literal>string</literal>,
   and a given enumeration supports only a single type at a time (that is, no union of <literal>int|string</literal>).
   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 <link linkend="language.enumerations.constants">Enumeration constants</link>.
  </para>

  <para>
   The equivalent values may be a constant scalar expression.
   Prior to PHP 8.2.0, the equivalent values had to be literals or literal expressions.
   This means that constants and constant expressions were not supported.
   That is, <code>1 + 1</code> was allowed, but not <code>1 + SOME_CONST</code>.
   is not.
  </para>

  <para>
   Backed Cases have an additional read-only property, <literal>value</literal>, which is the value
   specified in the definition.
  </para>

  <programlisting role="php">
<![CDATA[
<?php

print Suit::Clubs->value;
// Prints "C"
?>
]]>
  </programlisting>

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

  <programlisting role="php">
<![CDATA[
<?php

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

  <para>
   Backed enums implement an internal <interfacename>BackedEnum</interfacename> interface,
   which exposes two additional methods:
  </para>

  <simplelist>
   <member>
    <literal>from(int|string): self</literal> will take a scalar and return the corresponding
    Enum Case. If one is not found, it will throw a <classname>ValueError</classname>. This is mainly
    useful in cases where the input scalar is trusted and a missing enum value should be
    considered an application-stopping error.
   </member>
   <member>
    <literal>tryFrom(int|string): ?self</literal> will take a scalar and return the
    corresponding Enum Case. If one is not found, it will return <literal>null</literal>.
    This is mainly useful in cases where the input scalar is untrusted and the caller wants
    to implement their own error handling or default-value logic.
   </member>
  </simplelist>

  <para>
   The <literal>from()</literal> and <literal>tryFrom()</literal> 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 <literal>from()</literal> on a
   string-backed enum (or vice versa) will result in a <classname>TypeError</classname>,
   as will a float in all circumstances. All other parameter types will throw a TypeError
   in both modes.
  </para>

  <programlisting role="php">
<![CDATA[
<?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;
?>
]]>
  </programlisting>

  <para>Manually defining a <literal>from()</literal> or <literal>tryFrom()</literal> method on a Backed Enum will result in a fatal error.</para>

  </sect1>

 <sect1 xml:id="language.enumerations.methods">
  <title>Enumeration methods</title>

  <para>
   Enums (both Pure Enums and Backed Enums) may contain methods, and may implement interfaces.
   If an Enum implements an interface, then any type check for that interface will also accept
   all cases of that Enum.
  </para>

  <programlisting role="php">
<![CDATA[
<?php

interface Colorful
{
    public function color(): string;
}

enum Suit implements Colorful
{
    case Hearts;
    case Diamonds;
    case Clubs;
    case Spades;

    // Fulfills the interface contract.
    public function color(): string
    {
        return match($this) {
            Suit::Hearts, Suit::Diamonds => 'Red',
            Suit::Clubs, Suit::Spades => 'Black',
        };
    }

    // Not part of an interface; that's fine.
    public function shape(): string
    {
        return "Rectangle";
    }
}

function paint(Colorful $c)
{
   /* ... */
}

paint(Suit::Clubs);  // Works

print Suit::Diamonds->shape(); // prints "Rectangle"
?>
]]>
  </programlisting>

  <para>
   In this example, all four instances of <literal>Suit</literal> have two methods,
   <literal>color()</literal> and <literal>shape()</literal>. As far as calling code
   and type checks are concerned, they behave exactly the same as any other object instance.
  </para>

  <para>
   On a Backed Enum, the interface declaration goes after the backing type declaration.
  </para>

  <programlisting role="php">
   <![CDATA[
<?php

interface Colorful
{
    public function color(): string;
}

enum Suit: string implements Colorful
{
    case Hearts = 'H';
    case Diamonds = 'D';
    case Clubs = 'C';
    case Spades = 'S';

    // Fulfills the interface contract.
    public function color(): string
    {
        return match($this) {
            Suit::Hearts, Suit::Diamonds => 'Red',
            Suit::Clubs, Suit::Spades => 'Black',
        };
    }
}
?>
]]>
  </programlisting>

  <para>
   Inside a method, the <literal>$this</literal> variable is defined and refers to the Case instance.
  </para>

  <para>
   Methods may be arbitrarily complex, but in practice will usually return a static value or
   &match; on <literal>$this</literal> to provide
   different results for different cases.
  </para>

  <para>
   Note that in this case it would be a better data modeling practice to also define a
   <literal>SuitColor</literal> Enum Type with values Red and Black and return that instead.
   However, that would complicate this example.
  </para>

  <para>
   The above hierarchy is logically similar to the following class structure
   (although this is not the actual code that runs):
  </para>

  <programlisting role="php">
<![CDATA[
<?php

interface Colorful
{
    public function color(): string;
}

final class Suit implements UnitEnum, Colorful
{
    public const Hearts = new self('Hearts');
    public const Diamonds = new self('Diamonds');
    public const Clubs = new self('Clubs');
    public const Spades = new self('Spades');

    private function __construct(public readonly string $name) {}

    public function color(): string
    {
        return match($this) {
            Suit::Hearts, Suit::Diamonds => 'Red',
            Suit::Clubs, Suit::Spades => 'Black',
        };
    }

    public function shape(): string
    {
        return "Rectangle";
    }

    public static function cases(): array
    {
        // Illegal method, because manually defining a cases() method on an Enum is disallowed.
        // See also "Value listing" section.
    }
}
?>
]]>
  </programlisting>

  <para>
   Methods may be public, private, or protected, although in practice private and
   protected are equivalent as inheritance is not allowed.
  </para>

 </sect1>

 <sect1 xml:id="language.enumerations.static-methods">
  <title>Enumeration static methods</title>

  <para>
   Enumerations may also have static methods. The use for static methods on the
   enumeration itself is primarily for alternative constructors. E.g.:
  </para>

  <programlisting role="php">
<![CDATA[
<?php

enum Size
{
    case Small;
    case Medium;
    case Large;

    public static function fromLength(int $cm): self
    {
        return match(true) {
            $cm < 50 => self::Small,
            $cm < 100 => self::Medium,
            default => self::Large,
        };
    }
}
?>
]]>
  </programlisting>

  <para>
   Static methods may be public, private, or protected, although in practice private
   and protected are equivalent as inheritance is not allowed.
  </para>

 </sect1>

 <sect1 xml:id="language.enumerations.constants">
  <title>Enumeration constants</title>

  <para>
   Enumerations may include constants, which may be public, private, or protected,
   although in practice private and protected are equivalent as inheritance is not allowed.
  </para>

  <para>An enum constant may refer to an enum case:</para>

  <programlisting role="php">
<![CDATA[
<?php

enum Size
{
    case Small;
    case Medium;
    case Large;

    public const Huge = self::Large;
}
?>
]]>
  </programlisting>
 </sect1>

 <sect1 xml:id="language.enumerations.traits">
  <title>Traits</title>

  <para>Enumerations may leverage traits, which will behave the same as on classes.
   The caveat is that traits <literal>use</literal>d in an enum must not contain properties.
   They may only include methods, static methods, and constants. A trait with properties will
   result in a fatal error.
  </para>

  <programlisting role="php">
<![CDATA[
<?php

interface Colorful
{
    public function color(): string;
}

trait Rectangle
{
    public function shape(): string {
        return "Rectangle";
    }
}

enum Suit implements Colorful
{
    use Rectangle;

    case Hearts;
    case Diamonds;
    case Clubs;
    case Spades;

    public function color(): string
    {
        return match($this) {
            Suit::Hearts, Suit::Diamonds => 'Red',
            Suit::Clubs, Suit::Spades => 'Black',
        };
    }
}
?>
]]>
  </programlisting>
 </sect1>

 <sect1 xml:id="language.enumerations.expressions">
  <title>Enum values in constant expressions</title>

  <para>
   Because cases are represented as constants on the enum itself, they may be used as static
   values in most constant expressions: property defaults, static variable defaults, parameter
   defaults, global and class constant values. They may not be used in other enum case values, but
   normal constants may refer to an enum case.
  </para>

  <para>
   However, implicit magic method calls such as <classname>ArrayAccess</classname> on enums are not allowed in static
   or constant definitions as we cannot absolutely guarantee that the resulting value is deterministic
   or that the method invocation is free of side effects. Function calls, method calls, and
   property access continue to be invalid operations in constant expressions.
  </para>

  <programlisting role="php">
<![CDATA[
<?php

// This is an entirely legal Enum definition.
enum Direction implements ArrayAccess
{
    case Up;
    case Down;

    public function offsetExists($offset): bool
    {
        return false;
    }

    public function offsetGet($offset): mixed
    {
        return null;
    }

    public function offsetSet($offset, $value): void
    {
        throw new Exception();
    }

    public function offsetUnset($offset): void
    {
        throw new Exception();
    }
}

class Foo
{
    // This is allowed.
    const DOWN = Direction::Down;

    // This is disallowed, as it may not be deterministic.
    const UP = Direction::Up['short'];
    // Fatal error: Cannot use [] on enums in constant expression
}

// This is entirely legal, because it's not a constant expression.
$x = Direction::Up['short'];
var_dump("\$x is " . var_export($x, true));

$foo = new Foo();
?>
]]>
  </programlisting>
 </sect1>

 <sect1 xml:id="language.enumerations.object-differences">
  <title>Differences from objects</title>

  <para>
   Although Enums are built on classes and objects, they do not support all object-related functionality.
   In particular, Enum cases are forbidden from having state.
  </para>

  <simplelist>
   <member>Constructors and Destructors are forbidden.</member>
   <member>Inheritance is not supported. Enums may not extend or be extended.</member>
   <member>Static or object properties are not allowed.</member>
   <member>Cloning an Enum case is not supported, as cases must be singleton instances.</member>
   <member><link linkend="language.oop5.magic">Magic methods</link>, except for those listed below, are disallowed.</member>
   <member>Enums must always be declared before they are used.</member>
  </simplelist>

  <para>The following object functionality is available, and behaves just as it does on any other object:</para>

  <simplelist>
   <member>Public, private, and protected methods.</member>
   <member>Public, private, and protected static methods.</member>
   <member>Public, private, and protected constants.</member>
   <member>Enums may implement any number of interfaces.</member>
   <member>
    Enums and cases may have <link linkend="language.attributes">attributes</link> attached
    to them. The <constant>TARGET_CLASS</constant> target
    filter includes Enums themselves. The <constant>TARGET_CLASS_CONST</constant> target filter
    includes Enum Cases.
   </member>
   <member>
    <link linkend="object.call">__call</link>, <link linkend="object.callstatic">__callStatic</link>,
    and <link linkend="object.invoke">__invoke</link> magic methods
   </member>
   <member><constant>__CLASS__</constant> and <constant>__FUNCTION__</constant> constants behave as normal</member>
  </simplelist>

  <para>
   The <literal>::class</literal> magic constant on an Enum type evaluates to the type
   name including any namespace, exactly the same as an object. The <literal>::class</literal>
   magic constant on a Case instance also evaluates to the Enum type, as it is an
   instance of that type.
  </para>

  <para>
   Additionally, enum cases may not be instantiated directly with <literal>new</literal>, nor with
   <methodname>ReflectionClass::newInstanceWithoutConstructor</methodname> in reflection. Both will result in an error.
  </para>

  <programlisting role="php">
<![CDATA[
<?php

$clovers = new Suit();
// Error: Cannot instantiate enum Suit

$horseshoes = (new ReflectionClass(Suit::class))->newInstanceWithoutConstructor()
// Error: Cannot instantiate enum Suit
?>
]]>
  </programlisting>
 </sect1>

 <sect1 xml:id="language.enumerations.listing">
  <title>Value listing</title>

  <para>
   Both Pure Enums and Backed Enums implement an internal interface named
   <interfacename>UnitEnum</interfacename>. <literal>UnitEnum</literal> includes a static method
   <literal>cases()</literal>. <literal>cases()</literal> returns a packed array of
   all defined Cases in the order of declaration.
  </para>

  <programlisting role="php">
<![CDATA[
<?php

Suit::cases();
// Produces: [Suit::Hearts, Suit::Diamonds, Suit::Clubs, Suit::Spades]
?>
]]>
  </programlisting>

  <para>Manually defining a <literal>cases()</literal> method on an Enum will result in a fatal error.</para>
 </sect1>

 <sect1 xml:id="language.enumerations.serialization">
  <title>Serialization</title>

  <para>
   Enumerations are serialized differently from objects. Specifically, they have a new serialization code,
   <literal>"E"</literal>, that specifies the name of the enum case. The deserialization routine is then
   able to use that to set a variable to the existing singleton value. That ensures that:
  </para>

  <programlisting role="php">
<![CDATA[
<?php

Suit::Hearts === unserialize(serialize(Suit::Hearts));

print serialize(Suit::Hearts);
// E:11:"Suit:Hearts";
?>
]]>
  </programlisting>

  <para>
   On deserialization, if an enum and case cannot be found to match a serialized
   value a warning will be issued and &false; returned.</para>

  <para>
   If a Pure Enum is serialized to JSON, an error will be thrown. If a Backed Enum
   is serialized to JSON, it will be represented by its scalar value only, in the
   appropriate type. The behavior of both may be overridden by implementing
   <classname>JsonSerializable</classname>.
  </para>

  <para>For <function>print_r</function>, the output of an enum case is slightly different
   from objects to minimize confusion.
  </para>

  <programlisting role="php">
<![CDATA[
<?php

enum Foo {
    case Bar;
}

enum Baz: int {
    case Beep = 5;
}

print_r(Foo::Bar);
print_r(Baz::Beep);

/* Produces

Foo Enum (
    [name] => Bar
)
Baz Enum:int {
    [name] => Beep
    [value] => 5
}
*/
?>
]]>
  </programlisting>
 </sect1>

 <sect1 xml:id="language.enumerations.object-differences.inheritance">

  <title>Why enums aren't extendable</title>

  <simpara>
   Classes have contracts on their methods:
  </simpara>

  <programlisting role="php">
<![CDATA[
<?php

class A {}
class B extends A {}

function foo(A $a) {}

function bar(B $b) {
    foo($b);
}
?>
]]>
 </programlisting>

  <simpara>
   This code is type-safe, as B follows the contract of A, and through the magic of
   co/contra-variance, any expectation one may have of the methods will be
   preserved, exceptions excepted.
  </simpara>

  <simpara>
   Enums have contracts on their cases, not methods:
  </simpara>

  <programlisting role="php">
<![CDATA[
<?php

enum ErrorCode {
    case SOMETHING_BROKE;
}

function quux(ErrorCode $errorCode)
{
    // When written, this code appears to cover all cases
    match ($errorCode) {
        ErrorCode::SOMETHING_BROKE => true,
    };
}

?>
]]>
  </programlisting>

  <simpara>
   The &match; statement in the function <code>quux</code> can be static analyzed to cover
   all of the cases in ErrorCode.
  </simpara>

  <simpara>
   But imagine it was allowed to extend enums:
  </simpara>


  <programlisting role="php">
<![CDATA[
<?php

// Thought experiment code where enums are not final.
// Note, this won't actually work in PHP.
enum MoreErrorCode extends ErrorCode {
    case PEBKAC;
}

function fot(MoreErrorCode $errorCode) {
    quux($errorCode);
}

fot(MoreErrorCode::PEBKAC);

?>
]]>
 </programlisting>

  <simpara>
   Under normal inheritance rules, a class that extends another will pass
   the type check.
  </simpara>

  <simpara>
   The problem would be that the &match; statement in <code>quux()</code> no longer covers all
   the cases. Because it doesn't know about <code>MoreErrorCode::PEBKAC</code> the match will throw an exception.
  </simpara>

  <simpara>
   Because of this enums are final and can't be extended.
  </simpara>
 </sect1>

 <sect1 xml:id="language.enumerations.examples">
  &reftitle.examples;

  <para>
   <example>
    <title>Basic limited values</title>
    <programlisting role="php">
<![CDATA[
<?php

enum SortOrder
{
    case Asc;
    case Desc;
}

function query($fields, $filter, SortOrder $order = SortOrder::Asc)
{
     /* ... */
}
?>
]]>
    </programlisting>
    <para>
     The <literal>query()</literal> function can now proceed safe in the knowledge that
     <literal>$order</literal> is guaranteed to be either <literal>SortOrder::Asc</literal>
     or <literal>SortOrder::Desc</literal>. Any other value would have resulted in a
     <classname>TypeError</classname>, so no further error checking or testing is needed.
    </para>
   </example>
  </para>

  <para>

   <example>
    <title>Advanced exclusive values</title>

    <programlisting role="php">
<![CDATA[
<?php

enum UserStatus: string
{
    case Pending = 'P';
    case Active = 'A';
    case Suspended = 'S';
    case CanceledByUser = 'C';

    public function label(): string
    {
        return match($this) {
            self::Pending => 'Pending',
            self::Active => 'Active',
            self::Suspended => 'Suspended',
            self::CanceledByUser => 'Canceled by user',
        };
    }
}
?>
]]>
    </programlisting>

    <para>
     In this example, a user's status may be one of, and exclusively, <literal>UserStatus::Pending</literal>,
     <literal>UserStatus::Active</literal>, <literal>UserStatus::Suspended</literal>, or
     <literal>UserStatus::CanceledByUser</literal>. A function can type a parameter against
     <literal>UserStatus</literal> and then only accept those four values, period.
    </para>

    <para>
     All four values have a <literal>label()</literal> method, which returns a human-readable string.
     That string is independent of the "machine name" scalar equivalent string, which can be used in,
     for example, a database field or an HTML select box.
    </para>

    <programlisting role="php">
<![CDATA[
<?php

foreach (UserStatus::cases() as $case) {
    printf('<option value="%s">%s</option>\n', $case->value, $case->label());
}
?>
]]>
    </programlisting>
   </example>
  </para>

 </sect1>

 </chapter>

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