I've started trying out Go generics. Is there a way to define a generic type such as: one method does not need to be called with a pointer receiver, the other one needs to be called with a pointer receiver.
Example:
type MapKey interface {
comparable
}
type Unmarshaller[T any] interface {
UnmarshalJSON(b []byte) error
*T
}
type Marshaller[T any] interface {
MarshalJSON() ([]byte, error)
}
type MapValue[T any] interface {
Marshaller[T]
Unmarshaller[T]
}
type Map[T any, K MapKey, V MapValue[T]] struct {
_ K
_ V
}
func (m Map[K, V]) Save(k K, v V) error {
_, _ = v.MarshalJSON()
return nil
}
func (m Map[K, V]) Get(k K) (value V, err error) {
v := new(V)
err = v.UnmarshalJSON(m.bytesFromKey(k))
return *v, nil
}
// here I need to make sure V is actually a pointer so that UnmarshalJSON correctly propagates to the caller of GetTo and no copy is made of V
func (m Map[K, V]) GetTo(k K, v V) error {
bytesValue := m.kv.Get(m.bytesFromKey(k))
return v.UnmarshalJSON(v)
}
Error:
./map_generic.go:26:16: V has no constraints
./map_generic.go:27:11: v.MarshalJSON undefined (type bound for V has no method MarshalJSON)
./map_generic.go:31:16: V has no constraints
./map_generic.go:33:10: v.UnmarshalJSON undefined (type *V has no field or method UnmarshalJSON)
CodePudding user response:
I have not found a way to constrain one interface to have a pointer method and a non-pointer method. But I have found a way to have a struct type that has methods which accept either T or *T.
I had to split the interfaces (Marshaller, Unmarshaller) and constrain them to use the same type and then Map
type definition uses both.
This is the code (GOVERSION="devel go1.18-c239790 Sat Nov 13 03:33:55 2021 0000")
package storage
// Marshaller has T type parameter
type Marshaller[T interface{}] interface {
MarshalJSON() ([]byte, error)
}
// Unmarshaller has T type parameter and we force it to be a pointer
type Unmarshaller[T interface{}] interface {
UnmarshalJSON(b []byte) error
*T
}
// Map so here we need T, and we force Marshaller and Unmarshaller to be using the same type T
type Map[T any, K comparable, MV Marshaller[T], UV Unmarshaller[T]] struct {
}
func NewMap[T any, K comparable, MV Marshaller[T], UV Unmarshaller[T]]() Map[T, K, MV, UV] {
return Map[T, K, MV, UV]{}
}
// Save accepts the non pointer version of T to use for MarshalJSON
func (m Map[T, K, MV, UV]) Save(k K, v MV) {
v.MarshalJSON()
}
// GetTo accepts the pointer version of T to use for UnmarshalJSON
func (m Map[T, K, MV, UV]) GetTo(k K, v UV) {
v.UnmarshalJSON(nil)
}
This is the test file that asserts that we can force certain methods to accept pointers or not accept pointers of the same type.
package storage
import (
"testing"
)
type x struct {
}
func (x *x) UnmarshalJSON(b []byte) error {
return nil
}
func (x x) MarshalJSON() ([]byte, error) {
return nil, nil
}
// This will compile
func TestMarshalNoPointer(t *testing.T) {
m := NewMap[x, string, x, *x]()
m.Save("hi", x{})
}
// This will compile
func TestUnmarshalPointer(t *testing.T) {
m := NewMap[x, string, x, *x]()
m.GetTo("hi", &x{})
}
// This won't compile
func TestUnmarshalNoPointer(t *testing.T) {
m := NewMap[x, string, x, *x]()
m.GetTo("hi", x{})
}