In golang how calling a function with and without trailing brackets works when using net/http

Question

In the main function, I have a gorilla mux router, and a function to handle the route.

var router = mux.NewRouter()
func main() {   
    router.HandleFunc("/", ParseSlash)
    http.Handle("/", router)
    http.ListenAndServe(":8000", nil)
}

and ParseSlash looks like

const slash = `<h1>Login</h1>
<form method="post" action="/login">
  <label for="name">User name</label>
  <input type="text" id="name" name="name">
  <label for="password">Password</label>
  <input type="password" id="password" name="password">
  <button type="submit">Login</button>
</form>`

func ParseSlash(response http.ResponseWriter, request *http.Request)  {
    fmt.Fprintf(response, slash)
}

However, in main, we are not calling the function as ParseSlash(), but ParseSlash inside router.HandleFunc(). Where do the function get the parameters from, if we are not providing explicitely? What is this way of calling a function termed as?

Thank you.

Mr_Pink · Accepted Answer · 2016-06-13 15:22:29Z

11

You are not "calling" the function from main, you are providing it as a argument to HandleFunc, registering it to be called for the path "/" in the mux.Router. This pattern of providing a function to be called later is typically known as a "callback".

Your ParseSlash function is of the type http.HandlerFunc

type HandlerFunc func(ResponseWriter, *Request)

Your function is eventually called by the http.Server via its ServeHTTP method (here, via the mux.Router), passing the arguments shown. When the function is called, the http.ResponseWriter and *http.Request arguments are for the individual http request being handled.

edited Jun 13, 2016 at 15:22

answered Jun 13, 2016 at 14:59

Mr_Pink

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1 Comment

tsdtsdtsd Over a year ago

In addition, the "function without brackets" as you named it, is called a callback and is a common technique, for which you can find lots of resources online. It is very common (and imho essential) in e.g. JavaScript.

lofcek · Accepted Answer · 2016-06-13 15:44:07Z

2

It is a simple callback. You need that, when you want to call some function in the future, but now you don't have enough information to do it. Look - http.ListenAndServe create a server and waits for client.

You cannot call function ParseSlash, because it make sense after client will connect and will send address "/". When it happened router would have enough information to call your code back with arguments http.ResponseWriter and *http.Request.

Now you should learn how closures works - https://tour.golang.org/moretypes/25 . And you will done let's return back to http server https://www.nicolasmerouze.com/middlewares-golang-best-practices-examples/ .

answered Jun 13, 2016 at 15:44

lofcek

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