Dynamic JSON Decoding - go

Question

I have this function to parse HTTP results:
func (a *Admin) ResponseDecode(structName string, body io.Reader) (interface{}, error) {
content, err := ioutil.ReadAll(body)
if err != nil {
return nil, err
}
switch structName {
case "[]Cat":
var data []Cat
err = json.Unmarshal(content, &data)
if err != nil {
return nil, err
}
return data, err
case "[]Dog":
var data []Dog
err = json.Unmarshal(content, &data)
if err != nil {
return nil, err
}
return data, err
default:
log.Fatal("Can't decode " + structName)
}
return nil, nil
}
I do a type assertion after this method :
parsed, err := a.ResponseDecode("[]Cat", resp.Body)
if err != nil {
return nil, err
}
return parsed.([]Cat), nil
but how can I do to avoid the repetition of the code :
var data []Stuff
err = json.Unmarshal(content, &data)
if err != nil {
return nil, err
}
return data, err
Every time I add an object? Usually I would use generics, but this is Go. What's the good way to do that ?

Answer 1

You are passing in the name of the struct, and then expecting data of that type. Instead, you can simply pass the struct:
var parsed []Cat
err := a.ResponseDecode(&parsed, resp.Body)
where:
func (a *Admin) ResponseDecode(out interface{}, body io.Reader) error {
content, err := ioutil.ReadAll(body)
if err != nil {
return nil, err
}
return json.Unmarshal(content,out)
}
In fact, you can get rid of ResponseDecode function:
var parsed []Cat
err:=json.NewDecoder(body).Decode(&parsed)

Answer 2

Found what I was looking for here.
This function does the job :
func (a *Admin) ResponseDecode(body io.Reader, value interface{}) (interface{}, error) {
content, err := ioutil.ReadAll(body)
if err != nil {
return nil, err
}
err = json.Unmarshal(content, &value)
if err != nil {
return nil, err
}
return value, nil}