Function implementing interface

I want to know what is happening here.

There is the interface for a http handler:

type Handler interface {
    ServeHTTP(*Conn, *Request)
}

This implementation I think I understand.

type Counter int

func (ctr *Counter) ServeHTTP(c *http.Conn, req *http.Request) {
    fmt.Fprintf(c, "counter = %d\n", ctr);
    ctr++;
}

From my understanding it is that the type "Counter" implements the interface since it has a method that has the required signature. So far so good. Then this example is given:

func notFound(c *Conn, req *Request) {
    c.SetHeader("Content-Type", "text/plain;", "charset=utf-8");
    c.WriteHeader(StatusNotFound);
    c.WriteString("404 page not found\n");
}

// Now we define a type to implement ServeHTTP:
type HandlerFunc func(*Conn, *Request)
func (f HandlerFunc) ServeHTTP(c *Conn, req *Request) {
    f(c, req) // the receiver's a func; call it
}
// Convert function to attach method, implement the interface:
var Handle404 = HandlerFunc(notFound);

Can somebody elaborate on why or how these various functions fit together?

This:

type Handler interface {
    ServeHTTP(*Conn, *Request)
}

says that any type which satisfies the Handler interface must have a ServeHTTP method. The above would be inside the package http.

type Counter int

func (ctr *Counter) ServeHTTP(c *http.Conn, req *http.Request) {
    fmt.Fprintf(c, "counter = %d\n", ctr);
    ctr++;
}

This puts a method on the Counter type which corresponds to ServeHTTP. This is an example which is separate from the following.

> From my understanding it is that the
> type "Counter" implements the
> interface since it has a method that
> has the required signature.

That's right.

The following function by itself won't work as a Handler:

func notFound(c *Conn, req *Request) {
    c.SetHeader("Content-Type", "text/plain;", "charset=utf-8");
    c.WriteHeader(StatusNotFound);
    c.WriteString("404 page not found\n");
}

The rest of this stuff is just fitting the above so that it can be a Handler.

In the following, a HandlerFunc is a function which takes two arguments, pointer to Conn and pointer to Request, and returns nothing. In other words, any function which takes these arguments and returns nothing can be a HandlerFunc.

// Now we define a type to implement ServeHTTP:
type HandlerFunc func(*Conn, *Request)

Here ServeHTTP is a method added to the type HandlerFunc:

func (f HandlerFunc) ServeHTTP(c *Conn, req *Request) {
    f(c, req) // the receiver's a func; call it
}

All it does is to call the function itself (f) with the arguments given.

// Convert function to attach method, implement the interface:
var Handle404 = HandlerFunc(notFound);

In the above line, notFound has been finagled into being acceptable for the interface for Handler by artificially creating a type instance out of the function itself and making the function into the ServeHTTP method for the instance. Now Handle404 can be used with the Handler interface. It's basically a kind of trick.

What exactly don't you understand about the second half? It's the same pattern as above. Instead of defining the Counter type as an int, they define a function called notFound. They then create a type of function called HandlerFunc that takes two parameters, a connection and a request. they then create a new method called ServeHTTP, that gets bound to the HandlerFunc type. Handle404 is simply an instance of this class that uses the notFound function.