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

Question

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

Answer 1

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.