go-learning/3methods/interfaces.go

package main

import (
	"fmt"
	"math"
	"reflect"
)

type Abser interface {
	Abs() float64
}

type I interface {
	M()
}

type I2 interface {
	C()
}

type Vertex struct {
	X, Y float64
}

type Vertex2 struct {
	X, Y float64
}

type T struct {
	S string
}

type MyFloat float64

func main() {
	fmt.Println("An interface type is defined as a set of method signatures")

	var f Abser = MyFloat(1.2)
	var va Abser = &Vertex{3, 4}
	var v Abser = Vertex2{3, 4}
	// var v Abser = Vertex{3, 4} // ERROR, Same Vertex type not work because interface recognize them as same type

	fmt.Println(f.Abs())  // goes to `func (f MyFloat) Abs() float64 {`
	fmt.Println(va.Abs()) // goes to `func (v *Vertex) Abs() float64 {`
	fmt.Println(v.Abs())  // goes to `func (v Vertex2) Abs() float64 {`

	fmt.Println(reflect.TypeOf(f), reflect.TypeOf(f).Kind())
	fmt.Println(reflect.TypeOf(va), reflect.TypeOf(va).Kind())
	fmt.Println(reflect.TypeOf(v), reflect.TypeOf(v).Kind())

	/*
		TODO No need to interfaces ?
		```
		f := MyFloat(1.2)
		va := &Vertex{3, 4}
		v := Vertex2{3, 4}
		```
		Also this code block works even with Abser interface removed ?

		---

		Also in this example: https://github.com/Furkan-Gulsen/turkce-go-egitimi#interfaces

		There is no need to interfaces, just remove the interface and use methods normally as circle.area(), square.area() ...

		What is our motivation to use Interfaces ???
	*/

	// TODO what's the point! https://go.dev/tour/methods/10

	// TODO In this example describe function uses Interface as argument. And this prevents multiple describe function. Is it a use case for interfacse (https://go.dev/tour/methods/11)

	// TODO LOOK FOR HEAD-FIRST-GO EXAMPLES
	// LINK https://www.digitalocean.com/community/tutorials/how-to-use-interfaces-in-go
	// LINK https://duncanleung.com/understand-go-golang-interfaces/
	// LINK https://research.swtch.com/interfaces
	// TODO Understand interfaces more !

	fmt.Printf("\n\n")
	fmt.Println("If the concrete value inside the interface itself is nil, the method will be called with a nil receiver")
	var i I
	var t *T

	i = t
	describe(i)
	i.M()

	i = &T{"hello"}
	describe(i)
	i.M()

	fmt.Printf("\n")
	fmt.Println("A nil interface value holds neither value nor concrete type")

	var i2 I2
	describe2(i2)
	// i2.C() // panic: runtime error: invalid memory address or nil pointer dereference

	fmt.Printf("\n\n")
	fmt.Println("The interface type that specifies zero methods is known as the empty interface:")
	var i3 interface{}
	describe3(i3)
	fmt.Println("An empty interface may hold values of any type")

	i3 = 43
	describe3(i3)

	i3 = "hello"
	describe3(i3)

	fmt.Println("Empty interfaces are used by code that handles values of unknown type")

	fmt.Printf("\n\n")
	fmt.Println(" A type assertion provides access to an interface value's underlying concrete value. Can be used with t:=i.(T)")
	var i4 interface{} = "hello"

	s := i4.(string)
	fmt.Println(s)

	fmt.Println("You can also test if type is true with two variable. This also prevents panic")
	s, ok := i4.(string)
	fmt.Println(s, ok)

	f2, ok := i4.(float64)
	fmt.Println(f2, ok) // 0 false

	// f3 := i4.(float64) // panic: interface conversion: interface {} is string, not float64
	// fmt.Println(f3)

	fmt.Printf("\n\n")
	fmt.Println("You can also use interfaces for getting a variable as argument and then deciding type of this variable. var.(type) returns interfaces type. And then you can check different types with switch-case")
	somevar(61)
	somevar("hello")

	fmt.Printf("\n\n")
	fmt.Println("One of the most ubiquitous interfaces is Stringer defined by the fmt package. A Stringer is a type that can describe itself as a string. he fmt package (and many others) look for this interface to print values.")

	/*
		type Stringer interface {
			String() string
		}
	*/

	fmt.Println("This means you can change output style with String() method for a type")
	myVarA := Person{"Aliberk Sandıkçı", 50}
	myVarB := Person{"Burak", 100}
	myVarC := Person2{"Kemal", 150}

	fmt.Println(myVarA, myVarB, myVarC)

}

type Person struct {
	Name string
	Age  int
}

func (p Person) String() string {
	return fmt.Sprintf("%s + %d, ", p.Name, p.Age)
}

type Person2 struct {
	Name string
	Age  int
}

func (p Person2) String() string {
	return fmt.Sprintf("%s ! %d", p.Name, p.Age)
}

func somevar(i interface{}) {
	switch v := i.(type) {
	case int:
		fmt.Printf("INT: %v, %T\n", v, v)
	case string:
		fmt.Printf("STRING: %v, %T\n", v, v)
	default:
		fmt.Printf("OTHER: %v, %T\n", v, v)
	}
	// fmt.Println( i.(type) ) //	Can't be used outside of switches!
}

func (f MyFloat) Abs() float64 {
	if f < 0 {
		return float64(-f)
	}
	return float64(f)
}

func (v *Vertex) Abs() float64 {
	return math.Sqrt(v.X*v.X + v.Y*v.Y)
}

func (v Vertex2) Abs() float64 {
	return math.Sqrt(v.X*v.X+v.Y*v.Y) + 10
}

func (t *T) M() {
	if t == nil {
		fmt.Println("<nil>")
		return
	}
	fmt.Println(t.S)
}

func describe(i I) {
	fmt.Printf("(%v, %T)\n", i, i)
}

func describe2(i I2) {
	fmt.Printf("(%v, %T)\n", i, i)
}

func describe3(i interface{}) {
	fmt.Printf("(%v, %T)\n", i, i)
}