Problem accumulating/appending values in an array using recursion with Go - arrays

Question

First of all, this is my first non-dummy program using Go. Any recommendation will be appreciated.
Code description:
I want to retrieve all the information from an API where the information is being paginated. So I want to iterate through all the pages in order to get all the information.
This is what I did so far:
I have the these two functions:
func request(requestData *RequestData) []*ProjectsResponse {
client := &http.Client{
Timeout: time.Second * 10,
}
projects := []*ProjectsResponse{}
innerRequest(client, requestData.URL, projects)
return projects
}
func innerRequest(client *http.Client, URL string, projects []*ProjectsResponse) {
if URL == "" {
return
}
req, err := http.NewRequest("GET", URL, nil)
if err != nil {
log.Printf("Request creation failed with error %s\n", err)
}
req.Header.Add("Private-Token", os.Getenv("API_KEY"))
res, err := client.Do(req)
log.Printf("Executing request: %s", req.URL)
if err != nil {
log.Printf("The HTTP request failed with error %s\n", err)
}
data, _ := ioutil.ReadAll(res.Body)
var response ProjectsResponse
err = json.Unmarshal(data, &response)
if err != nil {
log.Printf("Unmarshalling failed with error %s\n", err)
}
projects = append(projects, &response)
pagingData := getPageInformation(res)
innerRequest(client, pagingData.NextLink, projects)
}
Undesired behavior:
The values in the projects []*ProjectsResponse array are being appended on each iteration of the recursion, but when the recursion ends I get an empty array list. So, somehow after the innerRequests ends, in the project variable inside the request method I get nothing.
Hope somebody and spot my mistake.
Thanks in advance.

Answer 1

I'm guessing that all of your project objects are scoped to the function so they no longer exist when the function ends. I don't think you need your projects to exist before you call innerRequest, so you should probably just have that method return the projects. I think something like this should work...
func innerRequest(client *http.Client, URL string) []*ProjectsResponse {
if URL == "" {
return nil
}
//More code...
pagingData := getPageInformation(res)
return append([]*ProjectsResponse{&response}, innerRequest(client, pagingData.NextLink)...)
}

Answer 2

The confusion lies in the way a slice is handled in Go. Here is the in-depth explanation, but I will abbreviate.
The common misconception is that the slice you pass around is a reference to the slice, which is false. The actual variable you operate on when you handle a slice is known as a slice header. This is a simple struct with a reference to the underlying array under the covers and follows Go's pass by value rules, i.e. it is copied when passed to a function. Thus, if it is not returned, you won't have the updated header.
Returning data from recursion follows a straightforward pattern. Here is a basic example. I'm also including a version that doesn't require a return, but operates on the slice as a reference, which will modify the original. (Note: Passing around slice pointers is generally not considered idiomatic Go)
Playground link: https://play.golang.org/p/v5XeYpH1VlF
// correct way to return from recursion
func addReturn(s []int) []int {
if finalCondition(s) {
return s
}
s = append(s, 1)
return addReturn(s)
}
// using a reference
func addReference(s *[]int) {
if finalCondition(*s) {
return
}
*s = append(*s, 1)
addReference(s)
}
// whatever terminates the recursion
func finalCondition(s []int) bool {
if len(s) >= 10 {
return true
}
return false
}