返回接口时的类型断言

I'm new to golang; however based on my current knowledge I understand that a value-type and a reference-type can both fulfill an interface. But it seems in regards to type assertion, how you return a struct does matter. See the following:

package main

import (
    "fmt"
)

type SomeError interface {
    Error() string
}

type ConcreteError struct{}

func (ConcreteError) Error() string {
    return "?"
}

func returnPointer() SomeError {
    return &ConcreteError{}
}

func returnVal() SomeError {
    return ConcreteError{}
}

func main() {
    pointer := returnPointer()
    value := returnVal()
    _, pointerWithPointer := pointer.(*ConcreteError);
    _, pointerWithValue := pointer.(ConcreteError);

    _, valueWithValue := value.(ConcreteError);
    _, valueWithPointer := value.(*ConcreteError)

    fmt.Printf("Returning a pointer, assert using (*ConcreteError): %v
", pointerWithPointer); // true
    fmt.Printf("Returning a pointer, assert using (ConcreteError): %v
", pointerWithValue); // false
    fmt.Printf("Returning a value,   assert using (ConcreteError): %v
", valueWithValue); // true
    fmt.Printf("Returning a value,   assert using (*ConcreteError): %v
", valueWithPointer); // false
}

So if my understanding is correct, the user needs to know how the struct is returned to correctly assert its type?

I'm going to guess and assume the standard practice in golang is to always return a pointer to a struct(i.e like *PathError)?

link to play: here

So if my understanding is correct, the user needs to know how the struct is returned to correctly assert its type?

It depends. If you need the type assertion to pass - you surely need to know the exact type of the value. a and *a are different types.

I'm going to guess and assume the standard practice in golang is to always return a pointer to a struct(i.e like *PathError)?

No.