根据switch语句更改变量类型

I am in the process of creating a POST HTTP request function in Go that will accept different data types through parameter but I am stuck when assigning value from switch statement to requestData variable.

Ideally, the requestData would be of nil type until we go to switch statement and then assign value and type to it. Any help appreciated :)

Error message on requestData: "syntax error: unexpected type, expecting type"

My code:

main() {
    ..

    // CASE 1: we are passing the form of url.Values type
    form := url.Values{}
    form.Add("note", "john2424")
    form.Add("http", "clear")

    response := POST("www.google.co.uk", client, form) // first POST request



    // CASE 2: we are passing the JSON data using []byte type
    jsonData := []byte(`{"ids":[12345]}`)
    response := POST("www.google.co.uk", client, jsonData) // second POST request
}

func POST(website string, client *http.Client, data interface{}) (bodyString string) {
    var requestData type // <<<<<<< Change requestData to a variable from switch case 

    switch data.(type) { // switch case based on type 
    case url.Values: // URL form data
        formattedData := data.(url.Values) // convert interface to url.Values
        requestData := strings.NewReader(formattedData.Encode()) // *Reader type
    case []byte: // JSON
        formattedData := data.([]byte) // convert interface to []byte
        requestData := bytes.NewBuffer(formattedData) // *Buffer type
    default: // anything else

    }

    request, err := http.NewRequest("POST", website, requestData)
    if err != nil {
        log.Fatal(err)
    }


    response, err := client.Do(request)
    if err != nil {
        log.Fatal(err)
    }

    defer response.Body.Close()
    body, err := ioutil.ReadAll(response.Body)
    if err != nil {
        log.Fatal(err)
    } else {
        bodyString = string(body)
    }

    return
}

get the typed data into the switch

typedData:=switch data.(type) {

then use that to convert for instance

case []byte: // JSON
        requestData := bytes.NewBuffer(typedData) // *Buffer type

Although @mkopriva 's playground example looks like a better idea