Interfaces + TODO/Questions!!!

3methods/interfaces.go

@@ -0,0 +1,216 @@
+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.Println("\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)
+}