added interfaces basics under go_basics module which teaches why interfaces are useful with examples

Author: anarchymonkey

go_basics/go.work

@@ -7,6 +7,7 @@ use (
 	./defer_panic_recover
 	./functions
 	./gomaps
+	./interfaces
 	./loops
 	./methods
 	./numeric_constants

go_basics/interfaces/go.mod

@@ -0,0 +1,3 @@
+module anarchymonkey.com/go-basics/interfaces
+
+go 1.20

go_basics/interfaces/interfaces.go

@@ -0,0 +1,84 @@
+package main
+
+import (
+	"fmt"
+	"math"
+)
+
+// An interface is a type which can hold any types that can implement the methods inside it
+
+// let us go through an example
+
+// Animal can be an interface, an animals
+
+type Shape interface {
+	calcArea() float64
+}
+
+type Square struct {
+	length int
+}
+
+type Circle struct {
+	radius int
+}
+
+type Rectangle struct {
+	Width  int
+	Height int
+}
+
+func printArea(s Shape) {
+	fmt.Println("The area is", s.calcArea())
+}
+
+func main() {
+	var s []Shape
+	var a Shape
+
+	var circle Circle = Circle{
+		radius: 25,
+	}
+
+	var square Square = Square{
+		length: 10,
+	}
+
+	// var rect Rectangle = Rectangle{
+	// 	Width:  10,
+	// 	Height: 100,
+	// }
+
+	var rect_v1 Rectangle = Rectangle{
+		Width:  100,
+		Height: 0,
+	}
+
+	// a = rect
+	a = &rect_v1
+
+	printArea(a)
+
+	// adding multiple shapes so that we can actually encapsulate the logic
+	s = []Shape{
+		&square,
+		&circle,
+	}
+
+	for _, shape := range s {
+		printArea(shape)
+	}
+
+}
+
+func (s *Square) calcArea() float64 {
+	return float64(s.length) * float64(s.length)
+}
+
+func (c *Circle) calcArea() float64 {
+	return float64(c.radius) * float64(c.radius) * math.Pi
+}
+
+func (r *Rectangle) calcArea() float64 {
+	return float64(r.Width) * float64(r.Height)
+}