Golang pass nil as optional argument to a function? - go

Question

In Golang, http.NewRequest has a specification like this:
func NewRequest(method, urlStr string, body io.Reader) (*Request, error)
However, I can pass nil as the body option if I don't want to pass the body to an io.Reader object, like this:
req, err := http.NewRequest("GET", "http://www.blahblah.org", nil)
How do I implement this functionality in my code? I have a function that I want to pass an optional string value so that it can page through API results however if I pass a nil to the string input I get this:
./snippets.go:32: cannot convert nil to type string
The parameters for my function look like this:
func getChallenges(after string) ([]challenge, string, error)

Answer 1

Go does not have "optional" arguments as a generally understood concept in other languages; nil is just the zero value for an interface (io.Reader in this case).
The equivalent zero value for a string is an empty string:
getChallenges("")
If you want to accept 0 or more of the same argument type, you use the variadic syntax:
func getChallenges(after ...string) ([]challenge, string, error)

Answer 2

You can modify you function to receive pointer value, like this:
func getChallenges(after *string) ([]challenge, string, error)
Then you can pass nil as an argument to it. But don't forget to check after for nil value inside your function before dereferencing it, or you will get a nil pointer exception:
func getChallenges(after *string) ([]challenge, string, error) {
if after == nil {
// No value specified
} else {
fmt.Printf("After: %s\n", *after) // Note pointer dereferencing with "*"
}
// ...
}
Another option:
Just use two functions:
func getChallenges(after string) {}
func getAllChallenges() {
return getChallenges(/* some default value here */)
}

Answer 3

you can use ellipse operator to send the optional parameters.. don't pass anything in optional parameter and check the length of parameter.
it should solve your problem
func foo(params ...int) {
fmt.Println(len(params))
}
func main() {
foo()
foo(1)
foo(1,2,3)
}

Answer 4

Maybe wrap it in a struct?
type NilableString struct {
value string;
}

Answer 5

You can use reflect.
In fact io.Reader is a interface.
So you can define signature like func getChallenges(after interface{}) ([]challenge, string, error)
interface{} is a empty interface, that is interface for anything.
But I suggest you use syntax args... to pass slice , refer to fmt.Printf implementation for usage, because if you pass no string the slice len is 0 and
this will avoid the reflect which I think too heavy for your function.