JSON marshalling with dynamic omitempty setting

main.go

package main

import (
	"encoding/json"
	"fmt"
)

type Nullable[T any] struct {
	val *T
}

func (n *Nullable[T]) MarshalJSON() ([]byte, error) {
	x, err := json.Marshal(n.val)
	return x, err
}

func (n *Nullable[T]) UnmarshalJSON(data []byte) error {
	var v T
	x := json.Unmarshal(data, &v)
	n.val = &v
	return x
}

type JsonStruct struct {
	StringFilled   *Nullable[string]   `json:"stringFilled,omitempty"`
	StringUnknown  *Nullable[string]   `json:"stringUnknown,omitempty"`
	StringNotShown *Nullable[string]   `json:"stringNotShown,omitempty"`
	ArrayFilled    *Nullable[[]string] `json:"arrayFilled,omitempty"`
	ArrayUnknown   *Nullable[[]string] `json:"arrayUnknown,omitempty"`
	ArrayNotShown  *Nullable[[]string] `json:"arrayNotShown,omitempty"`
}

func main() {
	var result JsonStruct

	filledString := "filled value"
	filledArray := []string{"a", "b", "c"}

	result.StringFilled = &Nullable[string]{val: &filledString} // normal value
	result.StringUnknown = &Nullable[string]{val: nil}          // value = null
	result.StringNotShown = nil                                 // will not appear at all
	result.ArrayFilled = &Nullable[[]string]{val: &filledArray}
	result.ArrayUnknown = &Nullable[[]string]{val: nil}
	result.ArrayNotShown = nil
	b, err := json.MarshalIndent(result, "", "  ")
	if err != nil {
		panic(err)
	}
	fmt.Printf("%s\n", b)

	var readStruct JsonStruct
	err = json.Unmarshal(b, &readStruct)
	if err != nil {
		panic(err)
	}
}