Basic Erlang Question

I have been trying to learn Erlang and came across some code written by Joe Armstrong:

start() ->
    F = fun interact/2,
    spawn(fun() -> start(F开发者_如何学C, 0) end).

interact(Browser, State) ->
    receive
        {browser, Browser, Str} ->
            Str1 = lists:reverse(Str),
            Browser ! {send, "out ! " ++ Str1},
            interact(Browser, State);
    after 100 ->
        Browser ! {send, "clock ! tick " ++ integer_to_list(State)},
        interact(Browser, State+1)
end.

It is from a blog post about using websockets with Erlang: http://armstrongonsoftware.blogspot.com/2009/12/comet-is-dead-long-live-websockets.html

Could someone please explain to me why in the start function, he spawns the anonymous function start(F, 0), when start is a function that takes zero arguments. I am confused about what he is trying to do here.

---

Further down in this blog post (Listings) you can see that there is another function (start/2) that takes two arguments:

start(F, State0) ->
{ok, Listen} = gen_tcp:listen(1234, [{packet,0},
                                  {reuseaddr,true},
                                  {active, true}]),
par_connect(Listen, F, State0).

The code sample you quoted was only an excerpt where this function was omitted for simplicity.

---

The reason for spawning a fun in this way is to avoid having to export a function which is only intended for internal use. One problem with is that all exported functions are available to all users even if they only meant for internal use. One example of this is a call-back module for gen_server which typically contains both the exported API for clients and the call-back functions for the gen_server behaviour. The call-back functions are only intended to be called by the gen_server behaviour and not by others but they are visible in the export list and not in anyway blocked.

Spawning a fun decreases the number of exported internal functions.

---

In Erlang, functions are identified by their name and their arity (the number of parameters they take). You can have more than one function with the same name, as long as they all have different numbers of parameters. The two functions you've posted above are start/0 and interact/2. start/0 doesn't call itself; instead it calls start/2, and if you take a look further down the page you linked to, you'll find the definition of start/2.

The point of using spawn in this way is to start a server process in the background and return control to the caller. To play with this code, I guess that you'd start up the Erlang interpreter, load the script and then call the start/0 function. This method would then start a process in the background and return so that you could continue to type into the Erlang interpreter.