iface.go

package main

import (
	"fmt"
	"reflect"
)

func main() {
	var m Foo = MyStruct{"test"}
	_ = m.DoSmth()
	fmt.Println()
	fmt.Println(reflect.TypeOf(m))
	fmt.Printf("%T", m)
}

type Foo interface {
	DoSmth() string
}

type MyStruct struct {
	Data string
}

func (m MyStruct) DoSmth() string {
	fmt.Printf("%v", m.Data)
	return m.Data
}