Why Is Unmarshal Failing With A Nested Struct? - json

Question

I am trying to retrieve information using Reddit's API. Here is some documentation on their json response, however, I got most of my information by just viewing the link in the browser and pretty-printing the response here.
The following code behaves as intended when the "Replies" field is commented out, but fails when it's not.
[edit] getData() is a function I wrote that uses Go's http Client to get a site response in bytes.
type redditThing struct {
Data struct {
Children []struct {
Data struct {
Permalink string
Subreddit string
Title string
Body string
Replies redditThing
}
}
}
}
func visitLink(link string) {
println("visiting:", link)
var comments []redditThing
if err := json.Unmarshal(getData(link+".json?raw_json=1"), &comments); err != nil {
logError.Println(err)
return
}
}
This throws the following error
json: cannot unmarshal string into Go struct field .Data.Children.Data.Replies.Data.Children.Data.Replies.Data.Children.Data.Replies of type main.redditThing
Any help would be greatly appreciated. Thank you all in advance!
[edit] here a link to some data causing the program to fail

Answer 1

The replies field can be the empty string or a redditThing. Fix by adding an Unmarshal function to handle the empty string:
func (rt *redditThing) UnmarshalJSON(data []byte) error {
// Do nothing if data is the empty string.
if bytes.Equal(data, []byte(`""`)) {
return nil
}
// Prevent recursion by declaring type x with
// same underlying type as redditThing, but
// with no methods.
type x redditThing
return json.Unmarshal(data, (*x)(rt))
}
The x type is used to prevent indefinite recursion. If the final line of the method is json.Unmarshal(data, rt), then json.Unmarshal function will call redditThing.UnmarshalJSON method which calls json.Unmarshal function and so on. Boom!
The statement type x redditThing declares a new type named x with the same underlying type as redditThing. The underlying type is a anonymous struct type. The underlying type has no methods, and crucially, the underlying type does not have the UnmarshalJSON method. This prevents recursion.