function used as value error while trying to wrap functions

Question

I'm teaching myself to use the net/http package using The Way to Go book. He mentions a way to wrap handling functions in a closure that takes care of panics like so:

func Index(w http.ResponseWriter, req *http.Request) {
    w.Header().Set("Content-Type", "text/html")
    fmt.Fprint(w, "<h2>Index</h2>")
}

func logPanics(function HandleFunc) HandleFunc {
    return func(w http.ResponseWriter, req *http.Request) {
        defer func() {
            if err := recover(); err != nil {
              log.Printf("[%v] caught panic: %v", req.RemoteAddr, err)
            }
        }()
        function(w, req)
    }
}

And then call http.HandleFunc using the above like so:

http.HandleFunc("/", logPanics(Index))

What I want to do is to "stack" more than one function to include more functionality. I wanted to add a closure that adds a mime type via .Header().Set(...) and I can call it like this:

func addHeader(function HandleFunc) HandleFunc {
    return func(w http.ResponseWriter, req *http.Request) {
        w.Header().Set("Content-Type", "text/html")
        function(w, req)
    }
}
(then in main())
http.HandleFunc("/", logPanics(addHeader(Index)))

But I thought it would be nice to shorten that whilst still keeping these functions separate using a wrapper function:

func HandleWrapper(function HandleFunc) HandleFunc {
    return func(w http.ResponseWriter, req *http.Request) {
        logPanics(addHeader(function(w, req)))
    }
}

But I get a function(w, req) used as value error. I haven't worked much with closures before and I feel like I'm definitely missing something here.

Thanks for the help!

James Henstridge · Accepted Answer · 2013-11-22 08:18:02Z

2

function(w, req) is a function call without a return value, while addHeader expects a function as its argument.

If you want to combine the two wrapper functions, you probably want something like this:

func HandleWrapper(function HandleFunc) HandleFunc {
    return logPanics(addHeader(function))
}

answered Nov 22, 2013 at 8:18

James Henstridge

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2 Comments

Protagonist Over a year ago

Thanks! That was it. As I understand it, the wrapper functions return function objects without executing the code - all the code gets executed when the function is actually called, right?

James Henstridge Over a year ago

Yep. So it isn't necessary to intercept the actual calls to the HandlerFunc in order to compose two wrappers.

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