How to create a top level object with no fields or methods in Golang?

Since I am from Java and new to Golang, I will try to explain what I want in Java.

    interface Car { }

    class MyCarA implements Car {
      int specificToA
    }

    class MyCarB implements Car {
      int specificToB
    }

I think such an interface (like Car) is called a marker interface in Java. It is just to indicate the compiler the necessary abstraction.

How can I do this in Golang ?

I have

type MyCarA struct {
   specificToA int
}
type MyCarB struct {
  specificToB int
}

How can I generalise these structs now ? Should it be an interface or another struct ?

You can do this:

type Car interface { IAmACar() }

type MyCarA struct {
  specificToA int
}
func (MyCarA) IAmACar() {}

type MyCarB struct {
  specificToB int
}
func (MyCarB) IAmACar() {}

You test for the marker using a type assertion:

_, itIsACar := v.(Car)

playground example

The Car interface can also be used to detect errors statically:

var c Car
c = MyCarA{0} // ok
c = 0 // error, int is not a car

The go/ast package does something similar. See uses of the function exprNode in the file ast.go.

The above method is correct & works for runtime detection, but it won't provide compile time detection. If you want compile time detection, pass the type to a function which takes the interface as an argument and check. See sample below:

package main

import "fmt"

type MyType struct {
	a int
	b int
}

type NotMyType struct {
	a int
	b int
}

type Printer interface {
	Print(a string) error
}

func (m *MyType) Print(s string) error {
	fmt.Println(m.a, m.b, s)
	return nil
}

//Uncomment following function to see compilation work
// func (m *NotMyType) Print(s string) error {
// 	fmt.Println(m.a, m.b, s)
// 	return nil
// }

func main() {
	t := &MyType{
		a: 1, b: 2,
	}

	t1 := &NotMyType{
		a: 1, b: 2,
	}

	checkPrintable(t)
	checkPrintable(t1)
}

func checkPrintable(p Printer) {
	p.Print("Test message")
}

To make it work you need to uncomment the Print function for NotMyType.

Hope this helps.