Combining records and enumerations

[Brian Goetz]

Yes, we considered exactly this during the design process of records.

There's nothing _wrong_ with this, and it's consistent with what records 
does with the component list.  But we concluded that it fell short of 
the "carries its weight" threshold.  Records acquire fields, accessors, 
ctors, equals, hashCode, toString, and soon, dtors.  But enums already 
have their own equals/hashCode/toString, and deconstruction makes less 
sense, so for enums it would be only fields and ctor, and maybe 
accessors.  So the benefit is less.  Add to that that there are many 
fewer enums than there are likely to be records, the benefit is even lesser.

Finally, we have deliberately held the corresponding syntactic position 
for classes uncolonized:

     class Foo(SOMETHING MIGHT GO HERE EVENTUALLY) { ... }

pending deeper thinking on what the right thing to do is.  There were 
many eager suggestions in the previous round, none of them seemed quite 
right, but its quite possible something good might emerge.  In which 
case it might make more sense for whatever goes in that position for 
enums, to be more like what classes do than like what records do.  So 
that's another reason to hold off doing anything special for enums.



On 10/12/2021 4:10 PM, Simon Ritter wrote:
> I hope this is the right list to post this to.
>
> I was recently using an enumeration and it struck me that enums 
> containing a constructor and member declarations could be simplified 
> by adopting some of the syntax from records.
>
> Here is an example of such an enumeration:
>
> public enum Planet {
>   MERCURY(0.4, 0.055),
>   VENUS(0.7, 0.815),
>   EARTH(1.0, 1.0),
>   MARS(0.107, 1.5);
>
>   private double orbitalDistance;
>   private double earthMass;
>
>   Planet(double orbitalDistance, double earthMass) {
>     this.orbitalDistance = orbitalDistance;
>     this.earthMass = earthMass;
>   }
>
>   public double orbitalDistance() {
>     return orbitalDistance;
>   }
>
>   public double earthMass() {
>     return earthMass;
>   }
> }
>
> Most of this boiler-plate code is identical to the code eliminated by 
> a record from a simple data class.  If we took the same approach we 
> could simplify this as:
>
> public enum Planet(double orbitalDistance, double earthMass) {
>   MERCURY(0.4, 0.055),
>   VENUS(0.7, 0.815),
>   EARTH(1.0, 1.0),
>   MARS(0.107, 1.5);
> }
>
> The grammar for enumerations would only need to be extended to include 
> an EnumHeader
>
> EnumDeclaration: {ClassModifier} enum TypeIdentifier EnumHeader 
> [ClassImplements] EnumBody
>
> with EnumHeader using the same format as RecordHeader
>
> EnumHeader:
>   ( [EnumComponentList] )
>
> I'm assuming changes to the compiler to implement this would not be 
> overly complex (although I have not looked at the code).
>
> I'd be very interested to hear other people's opinion on this.
>
> Regards,
>
> Simon.
>