难以理解一段golang代码

Question

package main

type Writeable interface {
    OnWrite() interface{}
}

type Result struct {
    Message string
}

func (r *Result) OnWrite() interface{} {
    return r.Message
}

// what does this line mean? what is the purpose?
var _ Writeable = (*Result)(nil)


func main() {

}

The comments in the code snippet expressed my confusion. As I understood, the line with comment notifies the compiler to check whether a struct has implemented the interface, but I am not sure very much. Could someone help explaining the purpose?

Answer 1

As you say it's a way to verify that Result implements Writeable. From the GO FAQ:

You can ask the compiler to check that the type T implements the interface I by attempting an assignment:

type T struct{} 
var _ I = T{}   // Verify that T implements I.

The blank identifier _ stands for the variable name which is not needed here (and thus prevents a "declared but not used" error).

(*Result)(nil) creates an uninitialized pointer to a value of type Result by converting nil to *Result. This avoids allocation of memory for an empty struct as you'd get with new(Result) or &Result{}.