How do interfaces work in Go? How do you use type assertions and type switches?

// Interface definition
type Shape interface {
    Area()      float64
    Perimeter() float64
}

// Circle satisfies Shape — no 'implements' keyword needed
type Circle struct{ Radius float64 }
func (c Circle) Area()      float64 { return math.Pi * c.Radius * c.Radius }
func (c Circle) Perimeter() float64 { return 2 * math.Pi * c.Radius }

type Rectangle struct{ W, H float64 }
func (r Rectangle) Area()      float64 { return r.W * r.H }
func (r Rectangle) Perimeter() float64 { return 2 * (r.W + r.H) }

// Polymorphic function — accepts any Shape
func printShape(s Shape) {
    fmt.Printf("Area=%.2f Perimeter=%.2f\n", s.Area(), s.Perimeter())
}

shapes := []Shape{Circle{5}, Rectangle{4, 6}}
for _, s := range shapes { printShape(s) }

// Type assertion — extract concrete type (safe form)
var s Shape = Circle{Radius: 3}
c, ok := s.(Circle)
if ok { fmt.Println("radius:", c.Radius) }  // radius: 3

// Type switch — check and handle multiple types
switch v := s.(type) {
case Circle:    fmt.Printf("circle r=%.1f\n", v.Radius)
case Rectangle: fmt.Printf("rect %.0fx%.0f\n", v.W, v.H)
default:        fmt.Println("unknown shape")
}

// Compile-time interface check (zero-cost assertion)
var _ Shape = Circle{}  // COMPILE ERROR if Circle doesn't implement Shape