This example shows how Go delegates covariance to the developer

covariance.go

package main

import (
	"fmt"
)

// Animal represents any animal on Earth.
type Animal interface {
	Noise() string
}

// Dog represents an animal that is often a pet.
type Dog struct{}

// Noise returns the noise a Dog makes.
func (Dog) Noise() string {
	return "Woof!"
}

// Print noises prints each animal's noise.
func PrintNoises(as []Animal) {
	for _, a := range as {
		fmt.Println(a.Noise())
	}
}

// Static type check that Dog is an Animal.
var _ Animal = Dog{}

// Bad static type check that []Dog is not equivalent to a []Animal.
// var _ []Animal = []Dog{}

func main() {
	// Program 1: This compiles and runs successfully.
	animals := []Animal{Dog{}}
	PrintNoises(animals)

	// Program 2: This does not compile and run successfully.
	// dogs := []Dog{Dog{}}
	// PrintNoises(dogs)

	// Program 3: This fixes the compile and run issues in program 2.
	// dogs := []Dog{Dog{}}

	// New logic: convert the slice of dogs to a slice of animals
	// animals := []Animal{}
	// for _, d := range dogs {
	//	animals = append(animals, Animal(d))
	// }
	// PrintNoises(animals)
}