Re: Let's discuss enums!

> Am 18.09.2015 um 01:06 schrieb Rowan Collins <rowan.collins@gmail.com>: > > Hi All, > > This has come up in passing a few times recently, but I'm not sure there's ever been > a dedicated discussion of it: would it be useful for PHP to have a built-in Enumeration type, and if > so, how should it look? > > Many other languages have enum types, with varying functionality. The central concept is always > that there's some declared type with a bunch of named constants, but beyond that there's a > lot of variability: for starters, is it a type of object, a special way of naming integers, or an > uncomparable type more like booleans and nulls? > > > Here are my thoughts on what I'd like from a PHP enum - I'm not using > "should" in a particularly strong sense, it's just a way of framing the points of > discussion: > > 1) enums should act like values, not mutable objects; an array-style copy-on-write behaviour > would be possible, but it feels wrong to me to store any state in an enum variable, so just plain > immutable would be my preference > > 2) the members of an enum should have a name which is accesible as a string, e.g. > Weekdays::SUNDAY->getName() == 'SUNDAY' > > 3) there should be no accessible "value" for a member; the value of Weekdays::SUNDAY > is simply Weekdays::SUNDAY, not 0 or 7 or 'SUNDAY' (I'm thinking that internally, > each member would be represented as an object pointer, so there's no real benefit to forcing > people to number everything like some languages do) > > 4) each enum member should be considered a singleton, in the sense that you can't > construct or destroy one, only reference them; all possible instances would be created as soon as > the enum was defined > > 5) following from (3) and (4), an enum value should compare equal only to itself, unlike in C# > for instance, where it would be comparable to an integer based on its numeric value; similarly it > shouldn't be possible to cast to or from an enum > > 6) an enum should be able to have additional fields; these would be initialised in the > enum's definition; this is inspired by Java and Python's ability to pass parameters to the > "constructor" of the enum, but it feels weird to me for any logic to happen in that > constructor other than simple assignments, so I'm thinking of a simpler syntax and > implementation. It also simplifies immutability if no userland code ever writes to the properties. > There may be an important use case for constructor logic I'm missing though? > > 7) an enum should have default static methods for accessing all the members of the enum as an > associative array > > 8) enums should be a first-class type, is_object(Weekdays::SUNDAY) should return false, for > instance; maybe Weekdays::SUNDAY instanceof Weekdays should return true though > > 9) additional static and instance methods should be definable, bearing in mind the immutability > constraints already discussed > > > Given the above, I think we might end up with something like this: > > enum Weekdays { > member MONDAY; // if there are no fields to initalise, the member just needs its name > declaring > member TUESDAY ( 2, 'Chewsdae' ); // positional arguments for populating fields in > the order they are defined; a bit like Java, but without the constructor > member WEDNESDAY { $dayNumber = 3, $sillyName = 'Wed Nose Day' }; // or maybe a > named-parameter syntax to make things clearer > member THURSDAY, FRIDAY, SATURDAY, SUNDAY; // don't force people to write each entry on > its own line; maybe even the "member" keyword is too much? > > public $dayNumber, $sillyName; // fields initialised for each member > > public static function getWeekend() { > return [ self::SATURDAY, self::SUNDAY ]; > } > > public function getZeroIndexedDayNumber() { > return $this->dayNumber - 1; > } > } > > $today = Weekdays::THURSDAY; > foreach ( Weekdays::getMembers() as $day_name => $day ) { > echo $day->dayNumber; > if ( $day == $today ) { > echo "Today!"; > } else { > echo $day_name; // equivalently: echo $day->getName(); > } > } > > // Do we need a static method to access a member by name, or is this good enough? > $favourite_day = Weekdays::getMembers()[ $_GET['fav_day'] ]; > > So, what are anyone's thoughts? Am I rambling on too much as usual? ;) > > Regards, > > -- > Rowan Collins > [IMSoP] I like enums in general, but I'd like to note that there's already a RFC in draft by Levi: https://wiki.php.net/rfc/enum <https://wiki.php.net/rfc/enum> As far as I know, the RFC is already fairly final and just lacks an implementation. So, I'd consider bikeshedding an actual RFC first. Bob