Enums as specified do not cover interop situations well

[DartBot]

This issue was originally filed by softwaredev...@gmail.com

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Enums as specified do not cover interop situations well where there's an expectation that an enum can be associated with a particular value (e.g. pnacl, http status messages with gaps in ranges, Windows messages, etc).

Please provide any additional information below.

Originally mentioned in comment to Issue #21416

[iposva-google]

Added Area-Language, Triaged labels.

[DartBot]

This comment was originally written by @mhausner

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Note that in this rare case, you can simply use symbolic integer constants, e.g. const int INTERNAL_SERVER_ERROR = 500.

[gbracha]

Enums were not intended to address this use case.

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Set owner to @gbracha.
Added Accepted label.

[DartBot]

This comment was originally written by googlegroups...@kaioa.com

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Enums in C++, C#, TypeScript, and D map to ints:

http://msdn.microsoft.com/en-us/library/2dzy4k6e.aspx
http://msdn.microsoft.com/en-us/library/sbbt4032.aspx
http://www.typescriptlang.org/Handbook#basic-types-enum
http://dlang.org/enum.html

Protobufs are similar:

https://developers.google.com/protocol-buffers/docs/proto#enum

I'd prefer if Dart's enums were like that, too. If some external system considers MONDAY to be equal to 107 (real-word example), then you have to use that value, too. There is no way around it.

If you use constants, you lose the tooling benefits.

If you use metadata to glue the values to the enum, they get rather heavy and there are some glaring issues:

    import 'dart:mirrors';

    class Enum {
      final values;
      const Enum(this.values);
    }
    
    @­Enum(const [11, 22, 33])
    enum Foo {UNO, DOS, TRES}
    
    @­Enum(const {
      Bar.UNO: 111,
      Bar.DOS: 222
    })
    enum Bar {UNO, DOS}
    
    int valueOf(e) {
      var values = reflect(e).type.metadata.first.reflectee.values;
      if (values is List) {
        return values[e.index];
      }
      return values[e];
    }
    
    main() {
      print(valueOf(Foo.DOS)); // 22
      print(valueOf(Bar.DOS)); // 222
    }