20181230-building-immutable-data-structures-in-go-counter-example.go

package main

import "fmt"

type Person struct {
	favoriteColors []string
}

func (p Person) WithFavoriteColorAt(i int, favoriteColor string) Person {
	p.favoriteColors = append(p.favoriteColors[:i],
		append([]string{favoriteColor}, p.favoriteColors[i+1:]...)...)

	return p
}

func main() {
	p := Person{
		favoriteColors: []string{"R", "G", "B"},
	}

	x := p.WithFavoriteColorAt(1, "xx")

	fmt.Println(p)
	fmt.Println(x)

	// Output:
	// {[R xx B]}
	// {[R xx B]}
}