Support Stringer interface on Optional[T]

Signed-off-by: moznion <moznion@mail.moznion.net>

option.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"encoding/json"
 	"errors"
+	"fmt"
 )
 
 var (
@@ -144,6 +145,18 @@ func (o Option[T]) IfNoneWithError(f func() error) error {
 	return f()
 }
 
+func (o Option[T]) String() string {
+	if o.IsNone() {
+		return "None[]"
+	}
+
+	v := o.Unwrap()
+	if stringer, ok := interface{}(v).(fmt.Stringer); ok {
+		return fmt.Sprintf("Some[%s]", stringer)
+	}
+	return fmt.Sprintf("Some[%v]", v)
+}
+
 // Map converts given Option value to another Option value according to the mapper function.
 // If given Option value is None, this also returns None.
 func Map[T, U any](option Option[T], mapper func(v T) U) Option[U] {

option_test.go

@@ -553,3 +553,18 @@ func TestOption_MarshalJSON_shouldHandleOmitemptyCorrectly(t *testing.T) {
 	assert.NoError(t, err)
 	assert.EqualValues(t, string(marshal), `{"normalVal":null}`)
 }
+
+type MyStringer struct {
+}
+
+func (m *MyStringer) String() string {
+	return "mystr"
+}
+
+func TestOption_String(t *testing.T) {
+	assert.Equal(t, "Some[123]", Some[int](123).String())
+	assert.Equal(t, "None[]", None[int]().String())
+
+	assert.Equal(t, "Some[mystr]", Some[*MyStringer](&MyStringer{}).String())
+	assert.Equal(t, "None[]", None[*MyStringer]().String())
+}