PHP RFC: Enumeration type (alternative proposal)

• Version: 0.9
• Date: 2020-05-14
• Author: Max Semenik, maxsem.wiki@gmail.com
• Status: Draft
• First Published at: http://wiki.php.net/rfc/enum_v2

Traditionally, PHP has used independent constants to represent related magic numbers. I propose to add a concept well known from many other languages, enumeration type.

Consider the following perfectly valid code:

preg_split($foo, $bar, LC_ALL * PHP_MAJOR_VERSION);

What will this call produce? I have no idea, either:)

It would be so much better to have a more foolproof and type-safe version, like:

preg_split($foo, $bar, Split::NoEmpty);

A simple enum:

enum Letters {
    a,          // Enum constants start with 0 by default
    b,          // 1, always previous value +1 if not specified explicitly
    c = 10,     // Values can be set explicitly
    d,          // 11
    e = 1,      // Constants with duplicate values are allowed
    f = 2 * 2,  // Can use the same expressions as class constants
    g = f,      // Can use other constants too
    h = g + 10, // And in expressions too
    H,          // Valid, constant names are case sensitive
    i,          // Optional comma after the last constant is permitted
}

A binary enum is used to represent a set of values:

binary enum FileMode {
    Read = 1,
    Write = 2,
    Execute = 1 << 2,
    ReadWrite = Read | Write,
}
 
$foo = FileMode::Read | FileMode::Execute;

Both enum types can extend other enums:

enum A { foo = 1 }
 
enum B extends A { bar = 2 }
 
$x = B::foo;

Overriding constants from base enums is not allowed:

enum C extends A {
    foo = 3 // CompileError
}

Constants must be int and thus fit into zend_long:

enum foo {
    bar = 2 ** 100, // CompileError
    baz = 1.5       // CompileError
}

Enums are implicitly coercible to bool and string:

function f(FileMode $mode) {
    if ($mode) {
        echo "mode: $mode";
    }
}
 
f(FileMode::Read); // Outputs "mode: 1"

Enums are immutable and don't support arithmetic operations:

$foo = FileMode::Read;
$foo = FileMode::Read + 1; // CompileError
$foo += 1;                 // TypeError
$bar = $foo + 1;           // TypeError

However, binary enums support bitwise operations:

$foo = FileMode::Read | FileMode::Execute;
$foo |= FileMode::Write;
$foo &= ~FileMode::Read;

Concrete enum types can be used as typehints:

function open(string $filename, FileMode $mode)

However, not the enum keyword itself:

function open(string $filename, enum $mode) // CompileError

When the type is clear from typehints, enum name can be omitted:

open('foo.txt', Read | Write)

is equivalent to:

open('foo.txt', FileMode::Read | FileMode::Write)

Same for switch statements:

function f(Letters $x) {
    switch ($x) {
    case a:
        // ...
    case b:
        // ...
    }
}

Internally, enums are classes and enum constants are public class constants. This makes them the fourth OOP-ey type in PHP, along with class, interface and trait. They can be autoloaded just like the former types. All enums inherit from this base class (here is PHP pseudocode):

final // In the sense that userspace can't explicitly extend it
class Enum {
    private int $value;
    private function __construct(); // It shouldn't be possible to create enums like this: $foo = new Enum();
    public function isBinary(): bool;
    public function __toString(): string {
        return (string)(int)$this->value;
    }
 
    // Whether the current value is represented by one of this enum's constants
    // or their combination for binary enums
    public function isKnownValue(): bool;
 
    // Returns a human readable representation of this enum's value
    // e.g. (FileMode::Read | FileMode::Write)->toHumanReadableString() would return 'Read | Write'
    public function toHumanReadableString(): string;
}

enum and binary will become reserved keywords. Class name 'Enum' (or whatever we decide to call it) will become unavailable.

PHP 8.1?

Make sure there are no open issues when the vote starts!

List existing areas/features of PHP that will not be changed by the RFC.

This helps avoid any ambiguity, shows that you have thought deeply about the RFC's impact, and helps reduces mail list noise.

This section details areas where the feature might be improved in future, but that are not currently proposed in this RFC.

* Accept this RFC (yes / no)? * What should be Enum fully qualified name (\Enum / \PHP\Enum)? * What enum constant naming convention should be used (PascalCase / camelCase / UPPER_UNDERSCORED)?

Links to any external patches and tests go here.

If there is no patch, make it clear who will create a patch, or whether a volunteer to help with implementation is needed.

Make it clear if the patch is intended to be the final patch, or is just a prototype.

For changes affecting the core language, you should also provide a patch for the language specification.

After the project is implemented, this section should contain

1. the version(s) it was merged into
2. a link to the git commit(s)
3. a link to the PHP manual entry for the feature
4. a link to the language specification section (if any)

Links to external references, discussions or RFCs * https://wiki.php.net/rfc/enum

Keep this updated with features that were discussed on the mail lists.