Returning interfaces in Golang - struct

Question

I am trying to write a method on a struct that takes in a interface type and returns that interface type but converted to the appropriate type.
type Model interface {
GetEntity()
}
type TrueFalseQuestions struct {
//some stuff
}
func (q *TrueFalseQuestions) GetEntity() {
//some stuff
}
type MultiQuestions struct {
//some stuff
}
func (q *MultiQuestions) GetEntity() {
//some stuff
}
type Manager struct {
}
func (man *Manager) GetModel(mod Model) Model {
mod.GetEntity()
return mod
}
func main() {
var man Manager
q := TrueFalseQuestions {}
q = man.GetModel(&TrueFalseQuestions {})
}
So when I call GetModel() with type TrueFalseQuestions I want to automatically return a TrueFalseQuestions type. I figured that would mean that my GetModel() method should return a Model type. That way if I pass a MultiQuestion type a MultiQuestion struct is returned.

Answer 1

You can't directly return a TrueFalseQuestions when the return type is Model. It will always be implicitly wrapped in a Model interface.
To get the TrueFalseQuestions back, you need to use a type-assertion. (you also need watch out for pointers vs values)
// this should be a pointer, because the interface methods all have pointer receivers
q := &TrueFalseQuestions{}
q = man.GetModel(&TrueFalseQuestions{}).(*TrueFalseQuestions)
That of course can panic if you got a MultiQuestions, so you should check the ok value, or use a type switch
switch q := man.GetModel(&TrueFalseQuestions{}).(type) {
case *TrueFalseQuestions:
// q isTrueFalseQuestions
case *MultiQuestions:
// q is MultiQuestions
default:
// unexpected type
}

Answer 2

You can't, however you can use type assertion on the returned value.
func main() {
var man Manager
tfq := &TrueFalseQuestions{}
q := man.GetModel(tfq)
if v, ok := q.(*TrueFalseQuestions); ok {
fmt.Println("v is true/false", v)
} else if v, ok := q.(*MultiQuestions); ok {
fmt.Println("v is mq", v)
} else {
fmt.Println("unknown", q)
}
}
playground