[Proposal]: abstract enumerations

[Youssef1313]

abstract enumerations

• Proposed
• Prototype: Not Started
• Implementation: Not Started
• Specification: Not Started

Summary

Allows for multiple enumerations to share common members.

Motivation

(excuse me for the example being very specific to Roslyn codebase)

This reduces extra clutter when enumerations are supposed to share common members (as an example, CodeAnalysis.CSharp.SyntaxKind and CodeAnalysis.VisualBasic.SyntaxKind). The current way to do things is to have a base class with abstract method, for example, IsIdentifier, where both overrides just do the same thing, return node.IsKind(SyntaxKind.IdentifierToken). This proposal removes this extra clutter by having a new concept, abstract enumerations.

Detailed design

Changes the syntax to:

enum_declaration
  : attribute_list* modifier* 'abstract'? 'enum' identifier_token base_list? '{' (enum_member_declaration (',' enum_member_declaration)* ','?)? '}' ';'?
  ;

• An abstract enum should be closed. i.e, a parameter of type MyAbstractEnum cannot be passed (MyAbstractEnum)0.
• A member of abstract enum cannot be assigned a value.
• A concrete enum should re-define all members in the abstract enum so each member is clear its value.
• A concrete enum may not derive from multiple enumerations, nor have a base type such as byte
• An abstract enum may have a base type such as byte, which implies that all derived enums are also byte.

abstract enum BaseEnum { CommonMember }

enum NotDerived : byte { } // the same as what it does today.
enum NotAllowed : byte, BaseEnum { } // Not allowed.

enum Derived : BaseEnum { CommonMember, OtherExtraMember }

Drawbacks

• Is this a common pattern enough to worth a new language feature?

Alternatives

Unresolved questions

Design meetings

[Youssef1313]

The proposal didn't make much sense when I tried to write a full sample. There is something missing for this to become useful.

@333fred I think it should be moved to a discussion in case someone can come up with a more formal, well-written one.