Go: Implementing a ManyDecode for a "set" of individual results

I have implemented a very simple Decode method (using gob.Decoder for now) - this works well for single responses - it would even work well for slices, but I need to implement a DecodeMany method where it is able to decode a set of individual responses (not a slice).

Working Decode method:

var v MyType
_ = Decode(&v)
...

func Decode(v interface{}) error {
   buf, _ := DoSomething() // func DoSomething() ([]byte, error)
   // error handling omitted for brevity
   return gob.NewDecoder(bytes.NewReader(buf)).Decode(v)
}

What I'm trying to do for a DecodeMany method is to deal with a response that isn't necessarily a slice:

var vv []MyType
_ = DecodeMany(&vv)
...

func DecodeMany(vv []interface{}) error {
   for _, g := range DoSomething() { // func DoSomething() []struct{Buf []bytes}
      
      // Use g.Buf as an individual "interface{}"
      // want something like:
      var v interface{} /* Somehow create instance of single vv type? */
      _ = gob.NewDecoder(bytes.NewReader(g.Buf)).Decode(v)
      vv = append(vv, v)
   }
   return
}

Besides not compiling the above also has the error of:
> cannot use &vv (value of type *[]MyType) as type []interface{} in argument to DecodeMany

If you want to modify the passed slice, it must be a pointer, else you must return a new slice. Also if the function is declared to have a param of type []interface{}, you can only pass a value of type []interface{} and no other slice types... Unless you use generics...

This is a perfect example to start using generics introduced in Go 1.18.

Change DecodeMany() to be generic, having a T type parameter being the slice element type:

When taking a pointer

func DecodeMany[T any](vv *[]T) error {
	for _, g := range DoSomething() {
		var v T
		if err := gob.NewDecoder(bytes.NewReader(g.Buf)).Decode(&v); err != nil {
			return err
		}
		*vv = append(*vv, v)
	}
	return nil
}

Here's a simple app to test it:

type MyType struct {
	S int64
}

func main() {
	var vv []MyType
	if err := DecodeMany(&vv); err != nil {
		panic(err)
	}
	fmt.Println(vv)
}

func DoSomething() (result []struct{ Buf []byte }) {
	for i := 3; i < 6; i++ {
		buf := &bytes.Buffer{}
		v := MyType{S: int64(i)}
		if err := gob.NewEncoder(buf).Encode(v); err != nil {
			panic(err)
		}
		result = append(result, struct{ Buf []byte }{buf.Bytes()})
	}
	return
}

This outputs (try it on the Go Playground):

[{3} {4} {5}]

When returning a slice

If you choose to return the slice, you don't have to pass anything, but you need to assign the result:

func DecodeMany[T any]() ([]T, error) {
	var result []T
	for _, g := range DoSomething() {
		var v T
		if err := gob.NewDecoder(bytes.NewReader(g.Buf)).Decode(&v); err != nil {
			return result, err
		}
		result = append(result, v)
	}
	return result, nil
}

Using it:

vv, err := DecodeMany[MyType]()
if err != nil {
	panic(err)
}
fmt.Println(vv)

Try this one on the Go Playground.