encoding

type Person struct {
	FirstName string `json:"firstName"`
	LastName  string `json:"lastName"`
	Age       int    `json:"age"`
}

decoder := NewJSONWriterDecoder[Person]("Person", func(p *Person) error {
	if p.FirstName == "" {
		return ErrFailedCheck
	}
	return nil
})

samplePerson := &Person{FirstName: "Tony", LastName: "Stark", Age: 53}

buf := new(bytes.Buffer)
_ = json.NewEncoder(buf).Encode(samplePerson)

_, _ = decoder.ReadFrom(buf)
p, err := decoder.Decode()
if err != nil {
	panic(err)
}
decodedPerson, ok := p.(*Person)
if !ok {
	panic("invalid type")
}

fmt.Printf("%s %s %d", decodedPerson.FirstName, decodedPerson.LastName, decodedPerson.Age)

Output:

Tony Stark 53