Challenge 1 — Solution

Task: Define an interface Speaker with a method Speak() string.
Define two structs, Dog and Cat, each with a Speak() method
returning an appropriate sound. Write a function announce(s Speaker)
that prints the result of Speak(), and call it with both a Dog and
a Cat.

package main

import "fmt"

type Speaker interface {
	Speak() string
}

type Dog struct{}

func (d Dog) Speak() string {
	return "Woof!"
}

type Cat struct{}

func (c Cat) Speak() string {
	return "Meow!"
}

func announce(s Speaker) {
	fmt.Println(s.Speak())
}

func main() {
	announce(Dog{})
	announce(Cat{})
}


Expected output:

Woof!
Meow!

Notes:
- Dog and Cat never mention Speaker anywhere in their definitions —
  they satisfy it automatically just by having a matching Speak()
  string method.
- struct{} (empty braces, no fields) is valid when a type doesn't
  need to store any data — Dog and Cat exist here purely to provide
  behaviour via Speak().
- announce works identically for both types because it only ever
  calls s.Speak(), never anything specific to Dog or Cat.