Why isn't fallthrough allowed in a type switch?

Question

I'm wondering why fallthrough isn't allowed in a type switch statement in golang.

According to the specification: "The "fallthrough" statement is not permitted in a type switch.", which doesn't explain much about WHY it isn't allowed.

The code attached is to simulate a possible scenario were a fallthrough in a type switch statement might have been useful.

Notice! This code doesn't work, it will produce the error: "cannot fallthrough in type switch". I'm just wondering what possible reasons might have been for not allowing the fallthrough statement in a type switch.

//A type switch question package main  import "fmt"  //Why isn't fallthrough in type switch allowed? func main() {     //Empty interface     var x interface{}      x = //A int, float64, bool or string value      switch i := x.(type) {     case int:         fmt.Println(i + 1)     case float64:         fmt.Println(i + 2.0)     case bool:         fallthrough     case string:         fmt.Printf("%v", i)     default:         fmt.Println("Unknown type. Sorry!")     } } 

Answer 1

How would you expect fallthrough to work? In this type switch, the i variable has a type that depends on the particular case that's invoked. So in the case bool the i variable is typed as bool. But in case string it's typed as string. So either you're asking for i to magically morph its type, which isn't possible, or you're asking for it to be shadowed by a new variable i string, which will have no value because its value comes from x which is not, in fact, a string.

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Here's an example to try and illustrate the problem:

switch i := x.(type) { case int:     // i is an int     fmt.Printf("%T\n", i); // prints "int" case bool:     // i is a bool     fmt.Printf("%T\n", i); // prints "bool"     fallthrough case string:     fmt.Printf("%T\n", i);     // What does that type? It should type "string", but if     // the type was bool and we hit the fallthrough, what would it do then? } 

The only possible solution would be to make the fallthrough cause the subsequent case expression to leave i as an interface{}, but that would be a confusing and bad definition.

If you really need this behavior you can already accomplish this with the existing functionality:

switch i := x.(type) { case bool, string:     if b, ok := i.(bool); ok {         // b is a bool     }     // i is an interface{} that contains either a bool or a string }