enum in Go

Edit (2023/03/25): Maybe after #57644 be resolved, we could use Rust enum in Go directly.

I provide a package to simulate Rust enum feature in Go. Also, you could see examples in the example folder.

But, it still has some problems:

• If defines two types with the same interface (e.g. SumTypeZero), then the type assertion would be ambiguous.

type Type1 enum.SumTypeZero
type Type2 enum.SumTypeZero

func CheckType(typ enum.SumType) {
    if _, ok := typ.(Type1); ok {
        ...
        return 
    }
    if _, ok := typ.(Type2); ok {
        ...
        return
    }
}

Because both of Type1 and Type2 implemented enum.SumTypeZero, so the CheckType cannot distinguish both of them.

• When defines two different types with same interface and with same type parameter(s), it still has the same issue.

type Type1 enum.SumTypeOne[int]
type Type2 enum.SumTypeOne[int]

func CheckType(typ enum.SumType) {
    if _, ok := typ.(Type1); ok {
        ...
        return 
    }
    if _, ok := typ.(Type2); ok {
        ...
        return
    }
}

In this implementation, enum.SumType is interface{}. As we know, any types in Go could be treated as interface{}.

IMO, we might could solve this problems, if we could:

• Treat named empty interface and unamed empty interface different.

// (O) compiled
// (X) not compiled

// named empty interface
type NamedEmpty interface {}

// unamed empty interface 
interface{}

func AcceptNameEmpty(NamedEmpty) {}
func AcceptUnamedEmpty(interface{}) {}

type implNameEmpty struct{
    NamedEmpty
    ...
}

type notImplNamedEmpty struct { 
    ... 
}

AcceptNameEmpty(implNameEmpty{})       // Case 1 (O)
AcceptNameEmpty(&implNameEmpty{})      // Case 2 (O)
AcceptNameEmpty(notImplNamedEmpty{})   // Case 3 (X)
AcceptUnamedEmpty(implNameEmpty{})     // Case 4 (O) backward compatible
AcceptUnamedEmpty(notImplNamedEmpty{}) // Case 5 (O)

• Treat named empty interfaces different.

// (O) compiled
// (X) not compiled

type NamedEmpty1 interface{}

type NamedEmpty2 interface{}

func AcceptNamedEmpty1(NamedEmpty1) {}

func AcceptNamedEmpty2(NamedEmpty2) {}

type implNamedEmpty1 struct{
    NamedEmpty1
    ...
}

type implNamedEmpty2 struct{
    NamedEmpty2
    ...
}

type implBothNamedEmpty12 struct {
    NamedEmpty1
    NamedEmpty2
    ...
}

AcceptNamedEmpty1(implNamedEmpty1{})       // Case 1  (O)
AcceptNamedEmpty1(&implNamedEmpty1{})      // Case 2  (O)
AcceptNamedEmpty1(implNamedEmpty2{})       // Case 3  (X)
AcceptNamedEmpty1(&implNamedEmpty2{})      // Case 4  (X)
AcceptNamedEmpty1(implBothNamedEmpty12{})  // Case 5  (O)
AcceptNamedEmpty1(&implBothNamedEmpty12{}) // Case 6  (O)

AcceptNamedEmpty2(implNamedEmpty1{})       // Case 7  (X)
AcceptNamedEmpty2(&implNamedEmpty1{})      // Case 8  (X)
AcceptNamedEmpty2(implNamedEmpty2{})       // Case 9  (O)
AcceptNamedEmpty2(&implNamedEmpty2{})      // Case 10 (O)
AcceptNamedEmpty2(implBothNamedEmpty12{})  // Case 11 (O)
AcceptNamedEmpty2(&implBothNamedEmpty12{}) // Case 12 (O)

• Treat named empty interface and type alias interface different.

// (O) compiled
// (X) not compiled

type NamedEmpty1 interface{}

type NamedEmpty2 NamedEmpty1

func AcceptNamedEmpty1(NamedEmpty1) {}

func AcceptNamedEmpty2(NamedEmpty2) {}

type implNamedEmpty1 struct{
    NamedEmpty1
    ...
}

type implNamedEmpty2 struct{
    NamedEmpty2
    ...
}

type implBothNamedEmpty12 struct {
    NamedEmpty1
    NamedEmpty2
    ...
}

AcceptNamedEmpty1(implNamedEmpty1{})       // Case 1  (O)
AcceptNamedEmpty1(&implNamedEmpty1{})      // Case 2  (O)
AcceptNamedEmpty1(implNamedEmpty2{})       // Case 3  (X)
AcceptNamedEmpty1(&implNamedEmpty2{})      // Case 4  (X)
AcceptNamedEmpty1(implBothNamedEmpty12{})  // Case 5  (O)
AcceptNamedEmpty1(&implBothNamedEmpty12{}) // Case 6  (O)

AcceptNamedEmpty2(implNamedEmpty1{})       // Case 7  (X)
AcceptNamedEmpty2(&implNamedEmpty1{})      // Case 8  (X)
AcceptNamedEmpty2(implNamedEmpty2{})       // Case 9  (O)
AcceptNamedEmpty2(&implNamedEmpty2{})      // Case 10 (O)
AcceptNamedEmpty2(implBothNamedEmpty12{})  // Case 11 (O)
AcceptNamedEmpty2(&implBothNamedEmpty12{}) // Case 12 (O)

In this proposal, which we didn't discuss:

1. tilde(~T) which I'm not so sure would it be impacted?