[RFC][goutil] Java style Optional construct

internal/goutil/optional.go

@@ -0,0 +1,93 @@
+package goutil
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+)
+
+var ErrNotPresent = errors.New("Optional value is not present")
+
+// Optional represents a value that may or may not be present.
+type Optional[T any] struct {
+	value T
+	isSet bool
+}
+
+// NewOptional creates a new Optional with the given value.
+func NewOptional[T any](value T) Optional[T] {
+	return Optional[T]{
+		value: value,
+		isSet: true,
+	}
+}
+
+// Empty returns an empty Optional.
+func Empty[T any]() Optional[T] {
+	return Optional[T]{}
+}
+
+// IsPresent returns true if the Optional contains a value.
+func (o Optional[T]) IsPresent() bool {
+	return o.isSet
+}
+
+// Get returns the value if present, or an error if not present.
+func (o Optional[T]) Get() (T, error) {
+	if !o.isSet {
+		var zero T
+		return zero, ErrNotPresent
+	}
+	return o.value, nil
+}
+
+// OrElse returns the value if present, or the given default value if not present.
+func (o Optional[T]) OrElse(defaultValue T) T {
+	if o.isSet {
+		return o.value
+	}
+	return defaultValue
+}
+
+// IfPresent calls the given function with the value if present.
+func (o Optional[T]) IfPresent(f func(T)) {
+	if o.isSet {
+		f(o.value)
+	}
+}
+
+// Map applies the given function to the value if present and returns a new Optional.
+func (o Optional[T]) Map(f func(T) T) Optional[T] {
+	if !o.isSet {
+		return Empty[T]()
+	}
+	return NewOptional(f(o.value))
+}
+
+// String returns a string representation of the Optional.
+func (o Optional[T]) String() string {
+	if !o.isSet {
+		return "Optional.Empty"
+	}
+	return fmt.Sprintf("Optional[%v]", o.value)
+}
+
+// UnmarshalJSON implements the json.Unmarshaler interface.
+func (o *Optional[T]) UnmarshalJSON(data []byte) error {
+	// Check if it's null first
+	if string(data) == "null" {
+		*o = Empty[T]()
+		return nil
+	}
+
+	// If not null, try to unmarshal into the value type T
+	var value T
+	err := json.Unmarshal(data, &value)
+	if err == nil {
+		*o = NewOptional(value)
+		return nil
+	}
+
+	// If it's neither a valid T nor null, return an error
+	return fmt.Errorf("cannot unmarshal %s into Optional[T]", string(data))
+}

internal/goutil/optional_test.go

@@ -0,0 +1,169 @@
+package goutil
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestOptional(t *testing.T) {
+	t.Run("NewOptional", func(t *testing.T) {
+		opt := NewOptional(42)
+		assert.True(t, opt.IsPresent())
+		value, err := opt.Get()
+		assert.NoError(t, err)
+		assert.Equal(t, 42, value)
+	})
+
+	t.Run("Empty", func(t *testing.T) {
+		opt := Empty[int]()
+		assert.False(t, opt.IsPresent())
+	})
+
+	t.Run("Get", func(t *testing.T) {
+		opt := NewOptional("test")
+		value, err := opt.Get()
+		assert.NoError(t, err)
+		assert.Equal(t, "test", value)
+
+		emptyOpt := Empty[string]()
+		_, err = emptyOpt.Get()
+		assert.Error(t, err)
+	})
+
+	t.Run("OrElse", func(t *testing.T) {
+		opt := NewOptional(10)
+		assert.Equal(t, 10, opt.OrElse(20))
+
+		emptyOpt := Empty[int]()
+		assert.Equal(t, 20, emptyOpt.OrElse(20))
+	})
+
+	t.Run("IfPresent", func(t *testing.T) {
+		opt := NewOptional(5)
+		called := false
+		opt.IfPresent(func(v int) {
+			called = true
+			assert.Equal(t, 5, v)
+		})
+		assert.True(t, called)
+
+		emptyOpt := Empty[int]()
+		emptyOpt.IfPresent(func(v int) {
+			t.Fail() // This should not be called
+		})
+	})
+
+	t.Run("Map", func(t *testing.T) {
+		opt := NewOptional(3)
+		mapped := opt.Map(func(v int) int { return v * 2 })
+		assert.True(t, mapped.IsPresent())
+		value, err := mapped.Get()
+		assert.NoError(t, err)
+		assert.Equal(t, 6, value)
+
+		emptyOpt := Empty[int]()
+		mappedEmpty := emptyOpt.Map(func(v int) int { return v * 2 })
+		assert.False(t, mappedEmpty.IsPresent())
+	})
+
+	t.Run("String", func(t *testing.T) {
+		opt := NewOptional("hello")
+		assert.Equal(t, "Optional[hello]", opt.String())
+
+		emptyOpt := Empty[string]()
+		assert.Equal(t, "Optional.Empty", emptyOpt.String())
+	})
+
+	t.Run("UnmarshalJSON", func(t *testing.T) {
+		var opt Optional[int]
+
+		err := json.Unmarshal([]byte("42"), &opt)
+		assert.NoError(t, err)
+		assert.True(t, opt.IsPresent())
+		value, err := opt.Get()
+		assert.NoError(t, err)
+		assert.Equal(t, 42, value)
+
+		err = json.Unmarshal([]byte("null"), &opt)
+		assert.NoError(t, err)
+		assert.False(t, opt.IsPresent())
+
+		err = json.Unmarshal([]byte(`"invalid"`), &opt)
+		assert.Error(t, err)
+	})
+}
+
+func TestOptionalUnmarshalJSONInStruct(t *testing.T) {
+	type TestStruct struct {
+		Name    string           `json:"name"`
+		Age     Optional[int]    `json:"age"`
+		Address Optional[string] `json:"address"`
+	}
+
+	t.Run("Present values", func(t *testing.T) {
+		jsonData := `{
+			"name": "John Doe",
+			"age": 30,
+			"address": "123 Main St"
+		}`
+
+		var result TestStruct
+		err := json.Unmarshal([]byte(jsonData), &result)
+
+		assert.NoError(t, err)
+		assert.Equal(t, "John Doe", result.Name)
+		assert.True(t, result.Age.IsPresent())
+		ageValue, err := result.Age.Get()
+		assert.NoError(t, err)
+		assert.Equal(t, 30, ageValue)
+		assert.True(t, result.Address.IsPresent())
+		addressValue, err := result.Address.Get()
+		assert.NoError(t, err)
+		assert.Equal(t, "123 Main St", addressValue)
+	})
+
+	t.Run("Missing optional values", func(t *testing.T) {
+		jsonData := `{
+			"name": "Jane Doe"
+		}`
+
+		var result TestStruct
+		err := json.Unmarshal([]byte(jsonData), &result)
+
+		assert.NoError(t, err)
+		assert.Equal(t, "Jane Doe", result.Name)
+		assert.False(t, result.Age.IsPresent())
+		assert.False(t, result.Address.IsPresent())
+	})
+
+	t.Run("Null optional values", func(t *testing.T) {
+		jsonData := `{
+			"name": "Bob Smith",
+			"age": null,
+			"address": "null"
+		}`
+
+		var result TestStruct
+		err := json.Unmarshal([]byte(jsonData), &result)
+
+		assert.NoError(t, err)
+		assert.Equal(t, "Bob Smith", result.Name)
+		assert.False(t, result.Age.IsPresent())
+		assert.True(t, result.Address.IsPresent())
+	})
+
+	t.Run("Invalid type for optional value", func(t *testing.T) {
+		jsonData := `{
+			"name": "Alice Johnson",
+			"age": "thirty"
+		}`
+
+		var result TestStruct
+		err := json.Unmarshal([]byte(jsonData), &result)
+
+		assert.Error(t, err)
+		assert.Contains(t, err.Error(), "cannot unmarshal")
+	})
+}