How Can Type Assertion Be Performed in Go with Unknown Interfaces?

Type Assertion in Go for Unknown Interfaces

The process of retrieving an object's value through reflection and performing type assertion to obtain the actual object using methods like:

obj := new(User)
out := reflect.ValueOf(obj).Elem().Interface().(User)
fmt.Println(out == *obj) // true

is well-known. However, challenges arise when the object's type is not known beforehand. Consider a function that accepts an unknown interface:

func Foo(obj interface{}) bool {
    // out := reflect.ValueOf(obj).Elem().Interface().( ... )
    return out == *obj
}

func main() {
    obj := new(User)
    fmt.Println(Foo(obj))
}

In the Foo function, determining the type of object being passed in is not possible. This poses the question: how can type assertion be performed in such scenarios?

The answer lies in the nature of type assertions. Type assertions allow leverage of static type checking in Go even when working with interfaces whose type cannot be verified statically. This functionality operates on the following principle:

Given a statically typed variable s of type t, the compiler ensures that s always remains of type t. It refuses compilation if s is used as a different type, as this would violate the established guarantee.

Now, imagine an interface variable i. Since its type cannot be known at compile-time, the compiler cannot guarantee that assigning i to s will not break the guarantee that s is of type t. Type assertions overcome this issue by stating, "We will verify at runtime if the types match, and perform the assignment only when they do." The compiler approves of this approach because it trusts that the assignment will only occur when the types are compatible, thus preserving the guarantee of s being of type t.

Essentially, this means the following happens at runtime:

if (i has type t) {
    s = i
} else {
    s = t{} // Zero value of t
}

The impossibility of the requested functionality arises from the need for the compiler to know the type against which to perform the check. Without this knowledge, the static type of s cannot be determined, rendering the verification process infeasible.

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