// you can play with the code via https://go.dev/play/p/bVfCe51O0en

package main

import "fmt"

// src/add.go

type Adder interface {
	add(int, int) int
}

type AddImpl struct{}

func (a *AddImpl) add(num1, num2 int) int {
	return num1 + num2
}

// src/main.go

func main() {
	fmt.Println("calling handler")
	res := myHandlerThatAlwaysReturns2(&AddImpl{})
	fmt.Println("response", res)

	// calling tests
	// this would typically done via `go test ...` but calling here to see output
	HandlerTest()
}

// src/handler.go

func myHandlerThatAlwaysReturns2(adder Adder) int {
	return (adder.add(1, 1))
}

// src/handler_test.go

type MockAddImpl struct{}

func (a *MockAddImpl) add(num1, num2 int) int {
	// you can return anything here
	return num1 - num2
}

func HandlerTest() {

	got := myHandlerThatAlwaysReturns2(&MockAddImpl{})
	want := 2

	if got != want {
		fmt.Println("ERROR")
	}

}