无法将nil转换为字符串类型

I have this piece of code in a middleware func:

params := r.URL.Query()

tok := params["x-huru-api-token"][0];

if tok == nil {
    tok := r.Header.Get("x-huru-api-token")
}

but I get this error:

anybody know what that's about?

The value nil is not a valid value for a string. If you want to fallback to the header on both missing values and empty values, then using the following code:

params := r.URL.Query()
tok := params.Get("x-huru-api-token")
if tok == "" {
    tok = r.Header.Get("x-huru-api-token")
}

If you only want to fallback to the header when the query parameter is missing, then use the following:

params := r.URL.Query()
var tok string
if values, ok := params["x-huru-api-token"]; ok && len(values) > 0 {
   tok = values[0]  // note that tok can be the empty string ""
} else {
   tok = r.Header.Get("x-huru-api-token")
}

Note one difference between the code in this answer and the question. This answer uses assignment to set tok inside the if statement. The code in the question uses a short variable declaration. The short variable declaration will not compile because the newly declared variable inside the if statement is not used.

String is not a pointer type in Go therefore it cannot be nil. Zero value for string is empty string ("").

The correct code should be:

params := r.URL.Query()

tok := params["x-huru-api-token"][0];

if tok == "" {
    tok := r.Header.Get("x-huru-api-token")
}