A value object is a language-agnostic pattern. So when we apply some sort of pattern to our codebase, we have to consider the characteristic of the language, and what problems the pattern should solve.
The requirements of a value object
These properties are required for a value object regardless of language specifications.
Value objects:
- should be immutable.
- should not be compared by identity, but by their structure.
- should encapsulate their behaviors with data.
In Go
Look at this example code.
package teaimport ( "errors" "strings")type Tea stringtype Cha struct { value string}func NewCha(v string) (Cha, error) { if !strings.HasSuffix(v, "cha") && !strings.HasSuffix(v, "chai") { return Cha{}, errors.New("Cha must end with `cha` or `chai`") } return Cha{value: v}, nil}func (c Cha) Value() string { return c.value}func (c Cha) HasAlcohol() bool { return false}package mainimport ( "fmt" "github.com/yuki-yamamura/gqlgen-practice/tea")func main() { // `tea.Tea` is just a named type; Go has no immutability for primitives. var assamTea tea.Tea = "assam tea" assamTea = "rosehip tea" fmt.Println(assamTea) // We cannot create a `Cha` object from other packages, but can do so via the `tea.NewCha` constructor. And it should be immutable. // bad := tea.Cha{value: "masara chai"} masaraChai, _ := tea.NewCha("masara chai") // masaraChai.value = "ocha" // The constructor has some validation logic. This code should return an error. _, err := tea.NewCha("rosehip tea") if err != nil { // ... } // A `tea.Cha` object has some behaviors; the `HasAlcohol` method is just like a property though. fmt.Println("Does masara chai have alcohol?", masaraChai.HasAlcohol()) // And we can compare two value objects. ocha, _ := tea.NewCha("ocha") fmt.Println("Are masara chai and ocha the same?", masaraChai == ocha)}
