Why is Fallthrough Prohibited in Go's Type Switches?

Fallthrough Prohibition in Type Switches:

In Go, the "fallthrough" statement is a language construct that allows the execution of subsequent cases in a switch statement without an explicit break statement. However, fallthrough is disallowed in type switches.

Why is this the case?

A type switch determines the type of an expression and assigns it to the variable specified in the switch case. This variable represents the specific type of the expression at that point in the switch.

If fallthrough were allowed, there would be a type mismatch issue. Let's consider the code snippet provided:

//Empty interface
var x interface{}

//A int, float64, bool or string value
x = ...

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!")
}

In case bool is true, the variable i is assigned the value of the bool within the interface x. If fallthrough were allowed, the execution would continue to the string case. However, at that point, the variable i would still hold the bool value. This type mismatch would result in an error.

Additionally, the fallthrough statement does not make sense in a type switch because it cannot change the type of the variable i. Type switches are intended to narrow down the type of the expression to a specific type at each case. Allowing fallthrough would defeat this purpose.

To achieve the desired behavior, one can use a switch statement with explicit type conversions within each case, as shown below:

switch i := x.(type) {
case bool, string:
    if b, ok := i.(bool); ok {
        fmt.Println(b) // value as bool
    } else {
        fmt.Printf("%v", i) // value as string
    }
default:
    fmt.Println("Unknown type. Sorry!")
}

By using this approach, one can handle a range of types within a single switch statement without the need for fallthrough.

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