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 '' | + | The most popular case of enumerations is '' |
===== Proposal ===== | ===== Proposal ===== | ||
Line 21: | 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 31: | 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 39: | 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 51: | 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 61: | Line 63: | ||
$a === $b; // true | $a === $b; // true | ||
- | </ | ||
- | Each Case class includes | + | $a instanceof Suit; // true |
- | + | $a instanceof | |
- | <code php> | + | |
- | print Suit::Clubs; | + | |
- | // prints " | + | |
</ | </ | ||
- | |||
- | That function may be overridden if desired. | ||
- | |||
- | [To Ilija: Do we want this part or not? I only thought of it while writing this. I don't know if it's good or bad.] | ||
- | |||
- | Enumerated type Cases may be used in union type definitions. | ||
- | |||
- | <code php> | ||
- | function gimmie_red_card(Suit:: | ||
- | </ | ||
- | |||
==== Enumerated Case Methods ==== | ==== Enumerated Case Methods ==== | ||
- | As both Enum Types and Enum Cases are implemented using classes, they may take 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. |
<code php> | <code php> | ||
- | |||
interface Colorful { | interface Colorful { | ||
public function color(): string; | public function color(): string; | ||
Line 120: | Line 106: | ||
} | } | ||
} | } | ||
+ | |||
+ | function paint(Colorful $c) { ... } | ||
+ | |||
+ | paint(Suit:: | ||
</ | </ | ||
+ | In this example, all four Enum cases will have a method '' | ||
- | In this example, all four Enum cases will have a method '' | + | Enum Cases may not implement |
- | Static methods on Cases are not supported. | + | Static methods on Cases are not supported. Static methods on the Enum Type are supported. |
- | Inside a method on a Case, The '' | + | (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.) |
- | (Note that in this case it would be a better data modeling practice to also define a '' | + | 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: | The above hierarchy is logically similar to the following class structure: | ||
<code php> | <code php> | ||
- | |||
interface Colorful { | interface Colorful { | ||
public function color(): string; | public function color(): string; | ||
Line 168: | Line 160: | ||
} | } | ||
</ | </ | ||
+ | ==== Value listing ==== | ||
+ | The enumeration itself has an automatically generated static method '' | ||
+ | |||
+ | <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 '' | ||
+ | |||
+ | A Primitive-Equivalent Case will automatically down-cast to its primitive when used in a primitive context. For example, when used with '' | ||
+ | |||
+ | <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 the primitive value in weak-typing mode, and produce a '' | ||
+ | |||
+ | A Primitive-Backed enumeration also has a static method '' | ||
+ | |||
+ | <code php> | ||
+ | $record = get_stuff_from_database($id); | ||
+ | print $record[' | ||
+ | // Prints " | ||
+ | $suit = Suit:: | ||
+ | $suit === Suit:: | ||
+ | </ | ||
+ | A Primitive-Backed enumeration additionally has a method '' | ||
+ | |||
+ | <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 ' | ||
+ | } | ||
+ | |||
+ | public function color(): string | ||
+ | { | ||
+ | // ... | ||
+ | } | ||
+ | } | ||
+ | </ | ||
==== Associated Values ==== | ==== Associated Values ==== | ||
- | Enumerated Cases may optionally include associated values. | + | 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. |
+ | |||
+ | Associated values are mutually exclusive with Primitive-Equivalent Cases. | ||
- | Bound values are defined using constructor property promotion. | + | Associated |
<code php> | <code php> | ||
Line 189: | Line 253: | ||
$my_walk === $next_walk; // FALSE! | $my_walk === $next_walk; // FALSE! | ||
</ | </ | ||
+ | Enum Cases may not implement a full constructor. However, they may list parameters that will be auto-promoted to properties using constructor promotion. The visibility modifier is required. Cases may not implement properties other than promoted properties. | ||
- | Enum Cases may not implement | + | An Enum Case that supports Associated Values is called an Associable Case. An Enum Case that does not have Associated Values is called |
- | An Enum Case that supports Associated Values is called an Associable Case. An Enum Case that does not have Associated Values is called | + | The Enum Type itself may not define associated values. Only a Case may do so. |
- | Associated values are always read-only, both internally to the class and externally. | + | 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. |
+ | |||
+ | On an Associable Case enumeration, | ||
Use cases that would require more complete class functionality (arbitrary properties, custom constructors, | Use cases that would require more complete class functionality (arbitrary properties, custom constructors, | ||
+ | |||
+ | ==== Match expressions ==== | ||
+ | |||
+ | When dealing with Unit Cases, '' | ||
+ | |||
+ | <code php> | ||
+ | $val = Suit:: | ||
+ | |||
+ | $str = match ($val) { | ||
+ | Suit:: | ||
+ | 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 ==== | ==== Examples ==== | ||
Line 214: | Line 307: | ||
} | } | ||
}; | }; | ||
- | | + | |
// This is an Associable Case. | // This is an Associable Case. | ||
- | case Some { | + | case Some(private mixed $value) |
// Note that the return type can be the Enum itself, thus restricting the return | // Note that the return type can be the Enum itself, thus restricting the return | ||
// value to one of the enumerated types. | // value to one of the enumerated types. | ||
Line 228: | Line 321: | ||
public function value(): mixed { | public function value(): mixed { | ||
// Still need to sort out match() for this to make sense. | // Still need to sort out match() for this to make sense. | ||
- | return match ($this) { | + | return match type ($this) { |
Optional:: | Optional:: | ||
Optional:: | Optional:: | ||
Line 235: | Line 328: | ||
} | } | ||
</ | </ | ||
+ | === State machine === | ||
+ | |||
+ | Enums make it straightforward to express finite state machines. | ||
+ | |||
+ | <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, the oven can be in one of three states (Off, On, and Idling, meaning the flame is not on, but it will turn back on when it detects it needs to). However, it can never go from Off to Idle or Idle to Off; it must go through On state first. That means no tests need to be written or code paths defined for going from Off to Idle, because it’s literally impossible to even describe that state. | ||
+ | |||
+ | (Additional methods are of course likely in a real implementation.) | ||
- | === Single | + | === Single |
Because all properties on an Enum are readonly, they offer a back-door way to create immutable objects. | Because all properties on an Enum are readonly, they offer a back-door way to create immutable objects. | ||
Line 250: | Line 367: | ||
$p->z = 9; // throws an Error of some kind, TBD. | $p->z = 9; // throws an Error of some kind, TBD. | ||
</ | </ | ||
- | |||
This is not a specific design goal of the implementation, | This is not a specific design goal of the implementation, | ||
- | |||
===== Backward Incompatible Changes ===== | ===== Backward Incompatible Changes ===== | ||
- | "enum" becomes a language | + | “enum” and “type” become |
- | + | ||
- | + | ||
- | ===== Proposed PHP Version(s) ===== | + | |
- | + | ||
- | Next PHP 8.x. | + | |
- | + | ||
- | + | ||
- | ===== RFC Impact ===== | + | |
- | + | ||
- | ===== Open Issues ===== | + | |
- | Make sure there are no open issues when the vote starts! | + | |
- | + | ||
- | ===== Unaffected PHP Functionality ===== | + | |
- | 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. | + | |
===== Future Scope ===== | ===== Future Scope ===== | ||
- | This section details areas where the feature might be improved in future, but that are not currently proposed in this RFC. | ||
- | ===== Proposed Voting Choices ===== | + | ==== Pattern matching |
- | This is a simple yes/no vote to include | + | 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 a single '' |
- | ===== Patches | + | For now, matching against the Enum Case and accessing properties directly (something not supported in most ADT-supporting languages) is “good enough” |
- | Links to any external patches | + | |
- | If there is no patch, make it clear who will create a patch, or whether a volunteer to help with implementation is needed. | + | ===== Voting ===== |
- | Make it clear if the patch is intended to be the final patch, or is just a prototype. | + | This is a simple yes/no vote to include Enumerations. 2/3 required to pass. |
- | For changes affecting the core language, you should also provide a patch for the language specification. | + | ===== References ===== |
- | ===== Implementation ===== | + | [Survey of enumerations supported by various languages, conducted by Larry](https:// |
- | After the project is implemented, this section should contain | + | |
- | - the version(s) it was merged into | + | |
- | | + | |
- | - a link to the PHP manual entry for the feature | + | |
- | - a link to the language specification section (if any) | + | |
- | + | ||
- | ===== References ===== | + | |
- | Links to external references, discussions or RFCs | + | |
- | ===== Rejected Features ===== | ||
- | Keep this updated with features that were discussed on the mail lists. |
rfc/enumerations_and_adts.1600536490.txt.gz · Last modified: 2020/09/19 17:28 by crell