I don't understand anything more now than I did at the start. It is clear we are talking past each other.
The values of Day are {SUNDAY, ..., SATURDAY} not {0, ..., 6}. We can, of course, establish a 1:1 mapping between those two sets, and the API provides a convenient forward mapping through the ordinal method and a somewhat less convenient reverse mapping through the values static method. However, at runtime, instances of Day are pointers not numbers, and ints outside the range [0, 6] will never be returned by the ordinal method and will cause IndexOutOfBoundsException if used like Day.values()[ordinal].
Tying back to purpose of this thread, Go cannot deliver the same guarantee. Even if we define
type Day int
const (
Sunday Day = iota
// ...
Saturday
)
then we can always construct Day(-1) or Day(7) and we must consider them in a switch statement. It is also trivial to cast to another "enum" type in Go, even if the variant doesn't exist on the other side. This sealed, nonconvertible nature of Java enums makes them "true" enums, which you can call tag-only discriminated unions or whatever if you want, but no such thing exists in Go. In fact, it is not even possible to directly adapt the Java approach, since sealed types of any kind, including structs, are impossible thanks to new(T) being allowed for all types T.
> This sealed, nonconvertible nature of Java enums makes them "true" enums, which you can call tag-only discriminated unions or whatever if you want, and no such thing exists in Go.
It is no secret that Go has a limited type system. In fact, upon release it was explicitly stated that their goal was for it to be a "dynamically-typed language with statically-typed performance", meaning that what limited type system it does have there only to support the performance goals. You'd have to be completely out to lunch while also living under a rock to think that Go has "advanced" types.
But, as before, enums are values. It is not clear why you want to keep going back to talking about type systems. That is an entirely different subject. It may be an interesting one, but off-topic as it pertains to this discussion specifically about enums, and especially not useful when talking in the context of Go which it isn't really intended to be a statically-typed language in the first place.
The values of Day are {SUNDAY, ..., SATURDAY} not {0, ..., 6}. We can, of course, establish a 1:1 mapping between those two sets, and the API provides a convenient forward mapping through the ordinal method and a somewhat less convenient reverse mapping through the values static method. However, at runtime, instances of Day are pointers not numbers, and ints outside the range [0, 6] will never be returned by the ordinal method and will cause IndexOutOfBoundsException if used like Day.values()[ordinal].
Tying back to purpose of this thread, Go cannot deliver the same guarantee. Even if we define
then we can always construct Day(-1) or Day(7) and we must consider them in a switch statement. It is also trivial to cast to another "enum" type in Go, even if the variant doesn't exist on the other side. This sealed, nonconvertible nature of Java enums makes them "true" enums, which you can call tag-only discriminated unions or whatever if you want, but no such thing exists in Go. In fact, it is not even possible to directly adapt the Java approach, since sealed types of any kind, including structs, are impossible thanks to new(T) being allowed for all types T.