Trouble understanding go function call syntax

I found this example here:

func(*myHandler) ServeHTTP(writer http.ResponseWriter, request *http.Request) {

    // What does this line do??
    if h, ok := route[request.URL.String()]; ok {
        h(writer, request)
        return
    }

    io.WriteString(writer, "my server: " + request.URL.String())
}

I am extremely puzzled by this line:

if h, ok := route[request.URL.String()]; ok { h(writer, request) }

first off, how can it be valid syntactically to declare ok after it is assigned to the result of route()?

second, if h is returned by route(), how can it be used in the definition of ok???

I am thoroughly confused by this. Gophers, please help.

Take a look at this page in Tour of Go. It explains that:

> Like for, the if statement can start with a short statement to execute before the condition.

Given your example:

> h, ok := route[request.URL.String()]; ok { ... }

The short statement is h, ok := route[request.URL.String()]. The ; after marks the end of the short statement. The ok that follows is the condition. If true, then the code block {...} is executed. In that code block you can use all the variables that were assigned in the short statement.

> first off, how can it be valid syntactically to declare ok after it is assigned to the result of route()?

ok is not declared after it is assigned. It is a boolean value, used in the condition.

> second, if h is returned by route(), how can it be used in the definition of ok`???

It's not definition of ok. It's the code block of the if statement.

Consider this alternative, almost equivalent writing style:

h, ok := route[request.URL.String()]
if ok {
    h(writer, request)
    return
}

This is perfectly clear, right? We just split the original code to two steps. But this is not the same. When writing this way, the h and ok variables are visible after the if statement. This is not desirable. It's a good practice to minimize the visibility of variables (also known as the live time). The longer a variable is visible, the longer the window of vulnerability during which it can be accidentally misused. This syntax of Go is truly great, genius, and I don't know a single other language where this kind of if statement is possible.

See also the section on the if statement in Effective Go.

Firstly: you should run through the Go Tour - specifically the section on Maps: https://tour.golang.org/moretypes/16

The "comma, ok" idiom here is designed to check that the key exists in the map. Doing a lookup on a non-existent key will panic (and likely crash your application).

if h, ok := route[request.URL.String()]; ok { h(writer, request) }

• If request.URL.String() exists, set h and set ok to true.
• If ok is true, then call the function h.
• Otherwise, write the URL to the response (and do nothing further).