Try It Yourself - SyntaxStudy Editor

HTML / CSS / JS

package main

import (
    "fmt"
    "math"
)

// Shape interface — any type with these methods satisfies it
type Shape interface {
    Area() float64
    Perimeter() float64
}

type Circle struct{ Radius float64 }
type Rect   struct{ Width, Height float64 }

func (c Circle) Area()      float64 { return math.Pi * c.Radius * c.Radius }
func (c Circle) Perimeter() float64 { return 2 * math.Pi * c.Radius }

func (r Rect) Area()        float64 { return r.Width * r.Height }
func (r Rect) Perimeter()   float64 { return 2 * (r.Width + r.Height) }

// printShape accepts any Shape — polymorphism without inheritance
func printShape(s Shape) {
    fmt.Printf("%T  area=%.2f  perimeter=%.2f
", s, s.Area(), s.Perimeter())
}

// Type switch on empty interface
func describe(v any) {
    switch t := v.(type) {
    case int:
        fmt.Printf("int: %d
", t)
    case string:
        fmt.Printf("string: %q
", t)
    case Shape:
        fmt.Printf("Shape with area %.2f
", t.Area())
    default:
        fmt.Printf("unknown type: %T
", t)
    }
}

func main() {
    shapes := []Shape{
        Circle{Radius: 5},
        Rect{Width: 4, Height: 6},
    }
    for _, s := range shapes {
        printShape(s)
    }

describe(42)
    describe("hello")
    describe(Circle{Radius: 3})
    describe(true)
}