Question

If types T1 and T2 are based on type T, and type T only comes into existence from a NewT1() or NewT2(), is there any way a function func (*T) WhoAmI() can know whether it "really" is a T1 or T2?

package main

import "fmt"
import "reflect"

type T struct{ s string }

func (v *T) WhoAmI() string {

    // pull type name with reflect
    fmt.Println(reflect.TypeOf(v).Elem().Name()) // always prints "T"!

    // todo: if I am actually T1
    return "T1"
    // todo: else if I am actually T2
    return "T2"
}

type T1 T

func NewT1(s string) T1 { return T1{s} }

type T2 T

func NewT2(s string) T2 { return T2{s} }

func main() {
    var t1 = T1{"xyz"}
    var t2 = T2{"pdq"}
    s1 := ((*T)(&t1)).WhoAmI() // would like to return "T1"
    s2 := ((*T)(&t2)).WhoAmI() // would like to return "T2"
    fmt.Println(s1, s2)
}

to speak technically:

once t1 type T1 is coerced into type T so func (*T) WhoAmI() can be called, does t1 completely lose the fact that its type is really T1? if not, how do we reclaim the knowledge from the perspective of a method receiving type T?

to speak generally:

in other words, if one type is based on another, if a variable of the derived type is coerced into the base type to run a method, can that method learn the real type of the receiver who called it?

Answer 1

No, it’s not possible. Creating a new type from an old one is not like creating a new class that inherits from a parent class in an class-based language. In your case T knows nothing about either T1 or T2 and if you’re calling the WhoAmI method you have a receiver of type T by definition.

Your design might work better with an interface. Try something more like this:

type T interface {
    WhoAmI() string
}

type T1 struct {
    s string
}

func (t *T1) WhoAmI() string { return "T1" }

type T2 struct {
    s string
}

func (t *T2) WhoAmI() string { return "T2" }

Try it on the Go playground

T1 and T2 both implement the interface T, so they can be used as type T.