[newbie] Erlangish way of Iterator pattern

Joe Armstrong (AL/EAB) joe.armstrong@REDACTED
Wed Jan 26 11:27:31 CET 2005

To solve problems like this I almost always use a module called tcp_server which is
available at


To use it call:

	tcp_server:start(Port, Fun, Max)

This starts a server which listens to Port up to Max simultaneous connections on Port are allowed.
The *first* time a connection is made to Port Then Fun(Socket) is called. 
Thereafter messages to the socket result in messages to the handler.

A typical server is usually written like this:

    start_server(Port) ->    
          S = self(),  
          process_flag(trap_exit, true), 
 				fun(Socket) -> input_handler(Socket, S) end, 

The loop() process is a central controller that all processes can use to synchronize amongst themselves if necessary It ends up as the variable "Controller" in the input_handler

A typical server is written like this:

  input_handler(Socket, Controller) ->
	{tcp, Socket, Bin} ->
	    gen_tcp:send(Socket, ...)
 	{tcp_closed, Socket} ->

 	Any ->

tcp_server can be used to build many different things. For example a web server or wiki system.

If you want to see how to make a web server using tcp_server read my second "spitting in the dust tutorial" on how to
make a web server.

This is at


This has all the code for a simple web server - the web server itself is split into a http parser (222 lines of code)
a server (104 lines) and the tcp_server (175) lines. 

I'll show you the code for the web server - it's like this:

server(Client, Master) ->
	{Client, closed} ->
	{Client, Request} ->
	    Response = generate_response(Request),
	    Client ! {self(), Response},
	    server(Client, Master)
    after 5000 ->

That's all :-)

The structure used in the web server provides a useful model of how to build these type of things.  It has

	- a connection handler (tcp_server) which manages tcp sessions
	- a device driver that parses HTTP messages and turns them into Erlang messages
	- a pure Erlang server

The general philosophy is "pretend that all things in the outside world are Erlang processes" - thus to program a web
server we model the browser as an Erlang process - it sends a {get, File} message to the server. The job of the
connection handler and device driver is to take the HTTP protocol message and turn them into Erlang terms.
Having done so, writing the server is a doodle.



> -----Original Message-----
> From: owner-erlang-questions@REDACTED 
> [mailto:owner-erlang-questions@REDACTED]On Behalf Of Gaspar 
> Chilingarov
> Sent: den 25 januari 2005 15:20
> To: erlang-questions@REDACTED
> Subject: [newbie] Erlangish way of Iterator pattern
> Hello!
> I'm beginning to program in erlang and hit the following problem.
> in mainstream language i will do the following -- create 
> Iterator class 
> to read data from socket or file
> i.e. class with Open/GetNext functions. Open should just do 
> all work for 
> preparing data source and GetNext will return lines of data or 
> EOF(freeing resources when reaching EOF).
> what i have in erlang -- i have erlang process, which can accept data 
> messages and eof messages and do processing. another process hangs 
> around, waiting for TCP connection and feeding first process 
> with data 
> read from TCP. in the same time i have shell script which runs once a 
> minute and feeds data from some file(or piped command line) 
> to TCP port.
> now i want to get rid of shell script, but i wish to have separate 
> module which will do this -- i.e. 1. running periodically external 
> command, 2. feeding to another process.
> are there any idioms in Erlang for this or it should be 
> implemented in 
> straight way -- i.e. just another process sitting there and 
> doing it's 
> job -- file reading/parsing/feeding to another process ?
> -- 
> Gaspar Chilingarov
> System Administrator
> t +3749 419763
> w www.web.am

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