Go does not provide union types at the language level, so we need some other way to represent a union of types. This is the best approach I’ve found so far.
package mainimport ( "fmt")// Use distinct struct types so that the compiler can tell them apart.type Aomori struct { Value string}type Chiba struct { Value string}type Ibaraki struct { Value string}// Use an interface to represent a union of types.type KantoPrefecture interface { Chiba | Ibaraki}type TohokuPrefecture interface { Aomori}// Use generics and a type switch to handle each type.func WelcomeToKantoPrefecture[T KantoPrefecture](p T) { var prefectureName string switch v := any(p).(type) { case Chiba: prefectureName = v.Value case Ibaraki: prefectureName = v.Value } fmt.Printf("Welcome to %s", prefectureName)}func main() { chiba := Chiba{ Value: "Chiba", } ibaraki := Ibaraki{ Value: "Ibaraki", } WelcomeToKantoPrefecture(chiba) WelcomeToKantoPrefecture(ibaraki) // aomori := Aomori{ // Value: "Aomori", // } // WelcomeToKantoPrefecture(aomori) // This would cause an `InvalidTypeArge` error.}
