Build fails when passing custom middleware type to alice.New() function

Question

This is about an error I am getting while I try to build my application.

I use Gorilla mux as a router and Alice for chaining the middlewares.

And I defined a custom type named 'Middleware' with the following signature;

type Middleware func(http.Handler) http.Handler

And following is the code where I chain the middlewares and the handler, using Alice.

if len(config.Middlewares()) > 0 {
   subRouter.Handle(config.Path(), alice.New(config.Middlewares()...).Then(config.Handler())).Methods(config.Methods()...).Schemes(config.Schemes()...)
}

But when I try to build, I get the following error in the console;

infrastructure/router.go:88:63: cannot use config.Middlewares() (type []Middleware) as type []alice.Constructor in argument to alice.New

I'd checked the code for alice.Constructor. It also has the same signature as my Middleware type.

I am using Go 1.13, and the following version of Alice.

github.com/justinas/alice v1.2.0

Can you please help me to sort out this?

Assignment or conversion between slices of different element types is not supported. Replace uses of your type `Middleware` with `alice.Constructor` or write a for loop to copy the elements from the `Middleware` slice to a new slice of `alice.Constructor`. — Charlie Tumahai, Feb 29 '20 at 18:07

Lucas Katayama · Accepted Answer · 2020-03-02T02:16:05.450

alice.Constructor has the same signature BUT it is defined as another type. So you can't just use it.

Watch this https://www.youtube.com/watch?v=Vg603e9C-Vg It has a good explanation.

What you can do is use type aliases Like this:

var Middleware = alice.Constructor

Will look like this:

Before:


func timeoutHandler(h http.Handler) http.Handler {
    return http.TimeoutHandler(h, 1*time.Second, "timed out")
}

func myApp(w http.ResponseWriter, r *http.Request) {
    w.Write([]byte("Hello world!"))
}

type Middleware func(http.Handler) http.Handler

func main() {
    middlewares := []Middleware{timeoutHandler}

    http.Handle("/", alice.New(middlewares...).ThenFunc(myApp))
}

After:

func timeoutHandler(h http.Handler) http.Handler {
    return http.TimeoutHandler(h, 1*time.Second, "timed out")
}

func myApp(w http.ResponseWriter, r *http.Request) {
    w.Write([]byte("Hello world!"))
}

type Middleware = alice.Constructor

func main() {
    middlewares := []Middleware{timeoutHandler}

    http.Handle("/", alice.New(middlewares...).ThenFunc(myApp))
}

Thanks, Lucas for the clarification and the Youtube link! – Bineeth C R Mar 01 '20 at 17:28 — Bineeth C R, Mar 01 '20 at 17:28

Build fails when passing custom middleware type to alice.New() function

1 Answers1