optional.go

package utils

import "encoding/json"

type Optional[T any] struct {
	explicit bool
	hasValue bool
	value    T
}

func (o *Optional[T]) UnmarshalJSON(b []byte) error {
	if len(b) == 0 {
		return nil
	}

	o.explicit = true
	if string(b) == "null" {
		return nil
	}

	o.hasValue = true
	return json.Unmarshal(b, &o.value)
}

func (o *Optional[T]) Explicit() bool {
	return o.explicit
}

func (o *Optional[T]) Value() *T {
	if !o.explicit || !o.hasValue {
		return nil
	}
	return &o.value
}

optional_test.go

package utils

import (
	"encoding/json"
	"testing"

	"github.com/stretchr/testify/assert"
	"github.com/stretchr/testify/require"
)

var (
	_ json.Unmarshaler = &Optional[int]{}
	_ json.Unmarshaler = &Optional[string]{}
)

type WithOptionals struct {
	IntField1 Optional[int]
	IntField2 Optional[int] `json:"int_field_2"`
}

func TestOptional(t *testing.T) {
	t.Run("Default should be implicit nulls", func(t *testing.T) {
		var data WithOptionals
		assert.False(t, data.IntField1.Explicit())
		assert.Nil(t, data.IntField1.Value())

		assert.False(t, data.IntField2.Explicit())
		assert.Nil(t, data.IntField2.Value())
	})
}

func TestOptional_Unmarshal(t *testing.T) {
	t.Run("Implicit null", func(t *testing.T) {
		var data WithOptionals
		const text = `{}`
		err := json.Unmarshal([]byte(text), &data)
		require.NoError(t, err)

		assert.False(t, data.IntField1.Explicit())
		assert.Nil(t, data.IntField1.Value())

		assert.False(t, data.IntField2.Explicit())
		assert.Nil(t, data.IntField2.Value())
	})

	t.Run("Explicit nulls", func(t *testing.T) {
		var data WithOptionals
		const text = `{
			"IntField1": null,
			"int_field_2": null
		}`
		err := json.Unmarshal([]byte(text), &data)
		require.NoError(t, err)

		assert.True(t, data.IntField1.Explicit())
		assert.Nil(t, data.IntField1.Value())

		assert.True(t, data.IntField2.Explicit())
		assert.Nil(t, data.IntField2.Value())
	})

	t.Run(`Explicit string "null" should fail`, func(t *testing.T) {
		var data WithOptionals
		const text = `{
			"IntField1": "null",
			"int_field_2": "null"
		}`
		err := json.Unmarshal([]byte(text), &data)
		require.Error(t, err)
	})

	t.Run("Happy scenario", func(t *testing.T) {
		var data WithOptionals
		const text = `{
			"IntField1": 123,
			"int_field_2": 456
		}`
		err := json.Unmarshal([]byte(text), &data)
		require.NoError(t, err)

		assert.True(t, data.IntField1.Explicit())
		assert.Equal(t, 123, *data.IntField1.Value())

		assert.True(t, data.IntField2.Explicit())
		assert.Equal(t, 456, *data.IntField2.Value())
	})
}