A function can accept an argument of an interface type. That interface type can be either

• Regular Interface

• Empty Interface

Let’s see example for both of them one by one

Regular Interface

A function can accept an argument of an interface type. Any type which implements that interface can be passed as an argument to that function.  Let’s understand with an example. Let’s say we have an animal interface as below

type animal interface {
    breathe()
    walk()
}

We have two struct types implementing that interface

type lion struct {
    age int
}

and

type dog struct {
     age int
}

In below code we have callBreathe and callWalk function which accept an argument of animal interface type. Both lion and dog instance can be passed to this function. We create an instance of both lion and dog type and pass it to the function. During compilation no type is checked while calling the function,  instead it is enough to check that the type passed to the function does implement breathe and walk method.

package main

import "fmt"

type animal interface {
	breathe()
	walk()
}

type lion struct {
     age int
}

func (l lion) breathe() {
	fmt.Println("Lion breathes")
}

func (l lion) walk() {
	fmt.Println("Lion walk")
}

type dog struct {
     age int
}

func (l dog) breathe() {
	fmt.Println("Dog breathes")
}

func (l dog) walk() {
	fmt.Println("Dog walk")
}

func main() {
	l := lion{age: 10}
	callBreathe(l)
	callWalk(l)

	d := dog{age: 5}
	callBreathe(d)
	callWalk(d)
}

func callBreathe(a animal) {
	a.breathe()
}

func callWalk(a animal) {
	a.breathe()
}

Output

Lion breathes
Lion walk
Dog breathes
Dog walk

Empty Interface

An empty interface has no methods , hence by default all types implement the empty interface. If you write a function that accepts an empty interface then you can pass any type to that function. See working code below

package main

import "fmt"

func main() {
    test("thisisstring")
    test("10")
    test(true)
}

func test(a interface{}) {
    fmt.Printf("(%v, %T)\n", a, a)
}

Output:

(thisisstring, string)
(10, string)
(true, bool)