options

options

cybozu-go/options provides Option[T], which represents an optional value of type T.

Example

opt := options.New(42) // or option.None[int]()

fmt.Println(opt) // prints "42"
fmt.Printf("%#v\n", opt) // prints "options.New(42)"

if opt.IsPresent() {
	// opt.Unwrap panics when opt is None.
	// When there are feasible default values, you can use UnwrapOr or UnwrapOrZero, which do not panic.
	v := opt.Unwrap()
	DoSomething(v)
}

// You can also use Unpack to check presence and get the value at once.
if v, ok := opt.Unpack(); ok {
	DoSomething(v)
}

Interoperability

• Option[T] can be serialized into or deserialized from JSON by encoding/json.
  • An Option[T] is serialized as if it is *T.
• Option[T] can be inserted into or selected from databases by database/sql.
  • Option[string] is handled as if it is sql.NullString, Option[time.Time] is handled as if it is sql.NullTime, and so on.
• Option[T] can be compared by google/go-cmp.
  • Option[T].Equal is implemented sololy for this purpose.

Compare complex values by go-cmp

Although Option[T] can be compared by go-cmp, it has some caveats when T is complex.

• cmp.Diff may be hard to read because diff of Option[T] is shown as diff of Option[T].String().
• cmp.Diff does not compare unexported fields of structs by default. On the other hand, Option[T].Equal is based on reflect.DeepEqual, which compares unexported fields.

You can use cmp.Transformer in such case.

type NestedData struct {
	Value string
}

type TestData struct {
	Value  string
	Nested *NestedData
}

func TestGoCmp(t *testing.T) {
	// Use *T instead of Option[T] in the cmp.Diff.
	cmpopt := cmp.Transformer("options.Option", options.Pointer[*TestData])

	d1 := options.New(&TestData{
		Value: "test",
		Nested: &NestedData{
			Value: "test",
		},
	})
	d2 := options.New(&TestData{
		Value: "test",
		Nested: &NestedData{
			Value: "test2",
		},
	})

	if diff := cmp.Diff(d1, d2, cmpopt); diff != "" {
		t.Errorf("diff:\n%s", diff)
	}
}