Wednesday, April 10, 2013

Enum Proposal for Dart


Gilad Bracha posted a proposal to add enums, one of the most requested features, to the Dart language. 



The contents from the post:


Rumor has it that people want enums in Dart. We've been looking at how to do this for a long time. We've examined all kinds of variations; the full account is, like the giant rat of Sumatra, a story the world is not yet ready to told (forgive me, Sherlock, but I couldn't resist). So, in response to popular demand (drumroll please):
We plan to introduce enumerated types after the 1.0 release. Here's the current proposal:
An enumerated type has the form:

enum E  = id1, … idn;

It is a top level declaration, and introduces a class E into the surrounding scope. E may not be subclassed, implemented, mixed in or instantiated. There are exactly n instances of E, available via the static getters E.id1, .., E.idn  and via top level constant variables of the same names. We may want to refine this so that enums support methods like >, >=, <, <=.

We expect to support iteration over enum values

  for (var e in E.values) { print(e.index); }

Where values is a list of all the enum instances, in the order they were declared.


Each enum instance has an ordinal value determined by its position in the sequence id1, … idn. The value can be retrieved via a getter index. It is thus possible to convert an integer into a enum of the corresponding ordinal value by indexing the list of values of the enum type

E.values[n] // should return the n’th enum

It should be possible to switch over enums, and we plan to issue warnings if the switch does not cover the entire set of enum values.

This is almost like sugar for

class E {
final index;
const E(this.index);
static const id1 = const E(0);
static const idn = const E(n - 1);
static const values = const <E>[id1 … idn];
}

const id1 = E.id1;
...
const idn = E.idn;


except for the additional support in switch, and constraints that E can not be instantiated or used as a mixin, superclass or superinterface.