rfc:enumerations_and_adts
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====== PHP RFC: Enumerations and Algebraic Data Types ====== | ====== PHP RFC: Enumerations and Algebraic Data Types ====== | ||
- | * Version: 0.9 | + | |
* Date: 2020-09-19 | * Date: 2020-09-19 | ||
* Author: Larry Garfield (larry@garfieldtech.com), | * Author: Larry Garfield (larry@garfieldtech.com), | ||
* Status: Draft | * Status: Draft | ||
- | * First Published at: http://wiki.php.net/ | + | * Target Version: PHP 8.1 |
+ | * Implementation: TBD | ||
+ | |||
+ | This RFC has been supplanted by [[rfc: | ||
+ | |||
+ | Please ignore this page. | ||
===== Introduction ===== | ===== Introduction ===== | ||
- | This RFC introduces Enumerations to PHP. Specifically, | + | This RFC introduces Enumerations to PHP. Specifically, |
- | Many languages have support for enumerations of some variety. | + | Many languages have support for enumerations of some variety. A [[https:// |
- | The specific implementation here draws inspiration primarily from Swift, Rust, and Kotlin, but is not (nor is it intended as) a perfect 1:1 port of any of them. | + | The specific implementation here draws inspiration primarily from Swift, Rust, and Kotlin, but is not (nor is it intended as) a perfect 1:1 port of any of them. Enumerations take many forms depending on the language, and we opted to implement the most robust combination of functionality feasible. Every piece of functionality described here exists in a similar form in at least one, usually several, other enumeration-supporting languages. It is implemented as a single RFC rather than a series of RFCs as the functionality all inter-relates, |
+ | The most popular case of enumerations is '' | ||
===== Proposal ===== | ===== Proposal ===== | ||
Line 20: | Line 26: | ||
==== Basic enumerations ==== | ==== Basic enumerations ==== | ||
- | This RFC introduces a new language construct, '' | + | This RFC introduces a new language construct, '' |
<code php> | <code php> | ||
Line 30: | Line 36: | ||
} | } | ||
</ | </ | ||
- | + | This declaration creates a new enumerated type named '' | |
- | This declaration creates a new enumerated type named %%Suit%%, which has four and only four legal values: '' | + | |
<code php> | <code php> | ||
Line 38: | Line 43: | ||
function pick_a_card(Suit $suit) { ... } | function pick_a_card(Suit $suit) { ... } | ||
- | pick_a_card($val); | + | pick_a_card($val); |
- | pick_a_card(Suit: | + | pick_a_card(Suit::Clubs); // OK |
- | pick_a_card(' | + | pick_a_card(' |
</ | </ | ||
- | + | In the simple case, multiple cases may be defined on a single line. The following is semantically equivalent to the definition above. | |
- | In the simple case, multiple cases may be defined on a single line. The following is semantically equivalent to the definition above. | + | |
<code php> | <code php> | ||
Line 50: | Line 54: | ||
} | } | ||
</ | </ | ||
- | |||
An Enumeration may have one or more '' | An Enumeration may have one or more '' | ||
- | Cases are not backed by a primitive value. | + | Cases are not backed by a primitive value. That is, '' |
<code php> | <code php> | ||
Line 60: | Line 63: | ||
$a === $b; // true | $a === $b; // true | ||
+ | |||
+ | |||
+ | $a instanceof Suit; // true | ||
+ | $a instanceof Suit:: | ||
</ | </ | ||
+ | ==== Enumerated Case Methods ==== | ||
+ | As both Enum Types and Enum Cases are implemented using classes, they may take methods. The Enum Type may also implement an interface, which all Cases must then fulfill, directly or indirectly. | ||
- | Each Case class includes | + | <code php> |
+ | interface Colorful { | ||
+ | public function color(): string; | ||
+ | } | ||
+ | |||
+ | enum Suit implements Colorful { | ||
+ | case Hearts { | ||
+ | public function color(): string { | ||
+ | return " | ||
+ | } | ||
+ | }; // Note the semi-colon here! | ||
+ | |||
+ | case Diamonds { | ||
+ | public function color(): string { | ||
+ | return " | ||
+ | } | ||
+ | }; | ||
+ | |||
+ | case Clubs { | ||
+ | public function color(): string { | ||
+ | return " | ||
+ | } | ||
+ | }; | ||
+ | |||
+ | case Spades { | ||
+ | public function color(): string { | ||
+ | return " | ||
+ | } | ||
+ | }; | ||
+ | |||
+ | public function shape(): string { | ||
+ | return " | ||
+ | } | ||
+ | } | ||
+ | |||
+ | function paint(Colorful $c) { ... } | ||
+ | |||
+ | paint(Suit:: | ||
+ | </ | ||
+ | In this example, all four Enum cases will have a method | ||
+ | |||
+ | Enum Cases may not implement interfaces themselves. | ||
+ | |||
+ | Static methods on Cases are not supported. Static methods on the Enum Type are supported. | ||
+ | |||
+ | (Ilija: We haven’t discussed static methods at all. This is what makes the most sense to me at the moment but we can easily revisit this. I’m flexible.) | ||
+ | |||
+ | Inside a method on a Case, The '' | ||
+ | |||
+ | (Note that in this case it would be a better data modeling practice to also define a '' | ||
+ | |||
+ | The above hierarchy is logically similar to the following class structure: | ||
<code php> | <code php> | ||
- | print Suit::Clubs; | + | interface Colorful { |
- | // prints | + | public function color(): string; |
+ | } | ||
+ | |||
+ | abstract class Suit implements Colorful { | ||
+ | public function shape(): string { | ||
+ | return " | ||
+ | } | ||
+ | } | ||
+ | |||
+ | class Hearts extends Suit { | ||
+ | public function color(): string { | ||
+ | return " | ||
+ | } | ||
+ | } | ||
+ | |||
+ | class Diamonds extends Suit { | ||
+ | public function color(): string { | ||
+ | return | ||
+ | } | ||
+ | } | ||
+ | |||
+ | class Clubs extends Suit { | ||
+ | public function color(): string { | ||
+ | return | ||
+ | } | ||
+ | } | ||
+ | |||
+ | class Spades extends | ||
+ | public function color(): string { | ||
+ | return | ||
+ | } | ||
+ | } | ||
</ | </ | ||
+ | ==== Value listing ==== | ||
- | That function may be overridden if desired. | + | The enumeration itself has an automatically generated static method '' |
- | ==== Enumerated Case Methods | + | <code php> |
+ | Suit:: | ||
+ | // Produces: [Suit:: | ||
+ | </ | ||
+ | ==== Primitive-Equivalent Cases ==== | ||
+ | By default, Enumerated Cases have no primitive 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 primitive (and thus trivially serializable) equivalent defined intrinsically is useful. | ||
+ | To define a primitive equivalent for an Enumeration, | ||
+ | <code php> | ||
+ | enum Suit: string { | ||
+ | case Hearts = ' | ||
+ | case Diamonds = ' | ||
+ | case Clubs = ' | ||
+ | case Spades = ' | ||
+ | } | ||
+ | </ | ||
+ | Primitive backing types of '' | ||
- | ===== Backward Incompatible Changes ===== | + | A Primitive-Equivalent Case will automatically down-cast to its primitive when used in a primitive context. For example, when used with '' |
- | "enum" | + | <code php> |
+ | print Suit:: | ||
+ | // prints | ||
+ | print "I hope I draw a " . Suit:: | ||
+ | // prints "I hope I draw a S". | ||
+ | </ | ||
+ | Passing a Primitive Case to a primitive-typed parameter or return will produce | ||
+ | A Primitive-Backed enumeration also has a static method '' | ||
- | ===== Proposed PHP Version(s) ===== | + | <code php> |
+ | $record | ||
+ | print $record[' | ||
+ | // Prints " | ||
+ | $suit = Suit:: | ||
+ | $suit === Suit:: | ||
+ | </ | ||
+ | A Primitive-Backed enumeration additionally has a method '' | ||
- | Next PHP 8.x. | + | <code php> |
+ | $list = Suit:: | ||
+ | $list === [ | ||
+ | ' | ||
+ | ' | ||
+ | ' | ||
+ | ' | ||
+ | ]; // true | ||
+ | </ | ||
+ | Primitive-backed Cases are not allowed to define a '' | ||
+ | <code php> | ||
+ | enum Suit: string { | ||
+ | case Hearts = ' | ||
+ | case Diamonds = ' | ||
+ | case Clubs = ' | ||
+ | case Spades = ' | ||
+ | public function color(): string { return ' | ||
+ | } | ||
- | ===== RFC Impact ===== | + | public function color(): string |
+ | { | ||
+ | // ... | ||
+ | } | ||
+ | } | ||
+ | </ | ||
+ | ==== Associated Values | ||
- | ===== Open Issues ===== | + | Enumerated Cases may optionally include associated values. An associated value is one that is associated with an instance of a Case. If a Case has associated values, it will **not** be implemented as a singleton. Each instance of the Case will then be its own object instance, so will not === another instance. |
- | Make sure there are no open issues when the vote starts! | + | |
- | ===== Unaffected PHP Functionality ===== | + | Associated values are mutually exclusive with Primitive-Equivalent Cases. |
- | List existing areas/ | + | |
- | This helps avoid any ambiguity, shows that you have thought deeply about the RFC's impact, and helps reduces mail list noise. | + | Associated values are defined using constructor property promotion. |
- | ===== Future Scope ===== | + | <code php> |
- | This section details areas where the feature might be improved in future, but that are not currently proposed in this RFC. | + | enum Distance { |
+ | case Kilometers(public int $num); | ||
+ | case Miles(public int $num); | ||
+ | } | ||
- | ===== Proposed Voting Choices ===== | + | $my_walk |
+ | // Named parameters work like any other function call. | ||
+ | $next_walk | ||
- | This is a simple yes/no vote to include Enumerations. | + | print $my_walk-> |
- | ===== Patches and Tests ===== | + | $my_walk |
- | Links to any external patches and tests go here. | + | </ |
+ | Enum Cases may not implement a full constructor. However, they may list parameters that will be auto-promoted | ||
- | If there is no patch, make it clear who will create | + | An Enum Case that supports Associated Values |
- | Make it clear if the patch is intended to be the final patch, or is just a prototype. | + | The Enum Type itself may not define associated values. Only a Case may do so. |
- | For changes affecting | + | Associated values are always read-only, both internally to the class and externally. Therefore, making them public does not pose a risk of 3rd party code modifying them inadvertently. They may, however, have attributes associated with them like any other property. |
- | ===== Implementation | + | On an Associable Case enumeration, |
- | After the project | + | |
- | | + | Use cases that would require more complete class functionality (arbitrary properties, custom constructors, |
- | - a link to the git commit(s) | + | |
- | - a link to the PHP manual entry for the feature | + | ==== Match expressions |
- | - a link to the language specification section | + | |
+ | When dealing with Unit Cases, '' | ||
+ | |||
+ | <code php> | ||
+ | $val = Suit:: | ||
+ | |||
+ | $str = match ($val) { | ||
+ | | ||
+ | Suit::Clubs => " | ||
+ | Suit:: | ||
+ | default => "The shape of my heart", | ||
+ | } | ||
+ | </ | ||
+ | That is not true when dealing with Associable Cases. Therefore, an alternate version of '' | ||
+ | |||
+ | <code php> | ||
+ | $val = Distance:: | ||
+ | |||
+ | $str = match type ($val) { | ||
+ | Distance:: | ||
+ | Distance:: | ||
+ | } | ||
+ | </ | ||
+ | (Ilija, your thoughts on this?) | ||
+ | |||
+ | ==== Examples ==== | ||
+ | |||
+ | Below are a few examples of Enums in action. | ||
+ | |||
+ | === Maybe === | ||
+ | |||
+ | The (in)famous Maybe Monad can be implemented like this: | ||
+ | |||
+ | <code php> | ||
+ | enum Maybe { | ||
+ | | ||
+ | case None { | ||
+ | public function bind(callable $f) { | ||
+ | return $this; | ||
+ | } | ||
+ | }; | ||
+ | |||
+ | // This is an Associable Case. | ||
+ | case Some(private mixed $value) { | ||
+ | // Note that the return type can be the Enum itself, thus restricting the return | ||
+ | // value to one of the enumerated types. | ||
+ | public function bind(callable $f) { | ||
+ | // $f is supposed to return a Maybe itself. | ||
+ | return $f($this-> | ||
+ | } | ||
+ | | ||
+ | |||
+ | // This method is available on both None and Some. | ||
+ | public function value(): mixed { | ||
+ | // Still need to sort out match() for this to make sense. | ||
+ | return match type ($this) { | ||
+ | Optional:: | ||
+ | Optional:: | ||
+ | }; | ||
+ | } | ||
+ | } | ||
+ | </ | ||
+ | === State machine === | ||
+ | |||
+ | Enums make it straightforward | ||
+ | |||
+ | <code php> | ||
+ | enum OvenStatus { | ||
+ | |||
+ | case Off { | ||
+ | public function turnOn() { return OvenStatus:: | ||
+ | }; | ||
+ | |||
+ | case On { | ||
+ | public function turnOff() { return OvenStatus:: | ||
+ | public function idle() { return OvenStatus:: | ||
+ | }; | ||
+ | |||
+ | case Idle { | ||
+ | public function on() { return OvenStatus:: | ||
+ | }; | ||
+ | } | ||
+ | </ | ||
+ | In this example, | ||
+ | |||
+ | (Additional methods are of course likely in a real implementation.) | ||
+ | |||
+ | === Single Associable Enums === | ||
+ | |||
+ | Because all properties on an Enum are readonly, they offer a back-door way to create immutable objects. | ||
+ | |||
+ | <code php> | ||
+ | enum Point { | ||
+ | | ||
+ | } | ||
+ | |||
+ | $p = Point:: | ||
+ | |||
+ | print $p->y; // prints 5 | ||
+ | $p->z = 9; // throws an Error of some kind, TBD. | ||
+ | </ | ||
+ | This is not a specific design goal of the implementation, | ||
+ | |||
+ | ===== Backward Incompatible Changes ===== | ||
+ | |||
+ | “enum” and “type” become language keywords, with the usual potential for naming conflicts with existing global constants. | ||
+ | |||
+ | ===== Future Scope ===== | ||
+ | |||
+ | ==== Pattern matching ==== | ||
+ | |||
+ | Most languages that have an equivalent of associated values also support pattern matching as a way to extract values from the Enum Case. Pattern matching allows | ||
+ | |||
+ | For now, matching against the Enum Case and accessing properties directly | ||
+ | |||
+ | ===== Voting ===== | ||
+ | |||
+ | This is a simple yes/no vote to include Enumerations. 2/3 required to pass. | ||
===== References ===== | ===== References ===== | ||
- | Links to external references, discussions or RFCs | ||
- | ===== Rejected Features ===== | + | [Survey of enumerations supported by various languages, conducted by Larry](https:// |
- | Keep this updated with features that were discussed on the mail lists. | + |
rfc/enumerations_and_adts.1600533298.txt.gz · Last modified: 2020/09/19 16:34 by crell