Why Does Type Assertion Panic When Casting to a Type Alias in Go?

Type Casting in Go: Understanding Type Assertions

In Go, casting is used to convert a value to a different data type. However, when dealing with type aliases, a slightly different approach is required, as demonstrated below.

The Code Snippet

Consider the following code:

type somethingFuncy func(int) bool

func funcy(i int) bool {
    return i%2 == 0
}

var a interface{} = funcy

func main() {

    _ = a.(func(int) bool)  // Works

    _ = a.(somethingFuncy)  // Panics
}

Here, the first type assertion (_ = a.(func(int) bool)) successfully converts the value of a to the function type func(int) bool. However, the second assertion (_ = a.(somethingFuncy)) fails with a panic.

Why the Panic?

The reason for the panic lies in the nature of type assertions in Go. For type assertions, only the actual type of the value matters. In this case, the type of a is interface{}, which is assigned the value of the function funcy. However, somethingFuncy is a distinct type alias, despite sharing the same function signature as func(int) bool.

Understanding Type Assertions

In Go, type assertions determine whether a value is of a specific type and, if so, extract the underlying value. The syntax is x.(T), where x is the value and T is the target type. The key here is that the target type must match the exact type of x, without any aliases.

Is There a Clean Way to Cast to a Longer Function Signature?

Unfortunately, there is no direct way to cast to a longer function signature using type assertions. This is because type assertions only consider the exact type of the value, not its compatibility with other types.

Conclusion

While type casting is essential in Go for converting values between different data types, it's important to understand the limitations when dealing with type aliases. Type assertions rely on exact type matches, and attempting to assert a value to a longer function signature will often result in a panic.

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