Erlang Programming/Processes

Erlang Processes and Messages
Processes are easy to create and control in Erlang.

The program chain_hello.erl builds a chain of processes as long as you like. Each process creates one process then sends a message to it. The program creates a chain of N processes which each print out "hello world!" (where  is some integer).

Processes send messages to and receive messages from one another. Messages are read with pattern matching. The messages are matched in a fifo (first in, first out) way.

Note 1: The order of the final output depends on process scheduling.

Note 2: Time flows downward (in each vertical line, see note 1).

This is a Process Message Diagram for the execution of: chain_hello:start(1). start(1) | spawns ---> listen(1) |                  |    |                spawns > listen(0) |                  |                            |    |                sends > speak --> prints --> "hello world 0" |                  |                            |  sends --> speak --> prints --> "hello world 1"     | |                           |                        ok                           ok

Program listing for: chain_hello.erl -module(chain_hello). -compile(export_all). % start(N)->                                                 % startup Pid1 = spawn(chain_hello, listen, [N]), Pid1 ! speak, io:format("done \n"). % listen(0)->                                                % base case receive speak -> io:format("hello world!~w\n", [0]) end; listen(N)->                                                % recursive case Pid2 = spawn(chain_hello, listen, [N-1]), Pid2 ! speak, receive speak -> io:format("hello world!~w\n", [N]) end.

% sample output  % % % 14> chain_hello:start(4). % done % hello world!4 % hello world!3 % hello world!2 % okhello world!1 % hello world!0