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Hello in Parallel
Parallel Hello World
-module(tree_hello). % 1
-export([start/0, speak/1]). % 2
% 3
start() -> % 4
Pid1 = spawn( tree_hello, speak,[ 1 ]), % 5
Pid2 = spawn( tree_hello, speak,[ 2 ]), % 6
Pid1 ! {hello, world}, % 7
Pid2 ! {hello, world}, % 8
done. % 9
% 10
speak(N) -> % 11
receive % 12
{hello, world} -> % 13
io:format("Hello, world! ~w \n", [N]) % 14
end. % 15
==========================================================================
output
--------------------
tree_hello:start().
hello world! 1
hello world! 2
done
Analysis of the example
Here is a simple hello world in the parallel spirit of erlang. The program, par_hello, will create 3 processes, one manager process called "start( )" and 2 worker processes called speak(1) and speak(2) in a tree like relationship. Start( ) creates speak(1) and speak(2), then start( ) sends a message to each worker. The message is {hello, world}. Each worker process responds by printing out "hello world". All three are running simultaneously when line 7 starts.
Lines 1 to 4: see serial "hello world".
Line 5 spawns a process called speak giving it one argument with the value 1.
Line 5 also creates a variable Pid1 and gives it the processes id number of speak(1).
Line 6 spawns a process called speak giving it one argument with the value 2.
Line 6 also creates a variable Pid2 and gives it the process id number of speak(2).
Line 7 uses the Pid1(process id number of speak(1) to send a message to speak(1).
Line 8 uses the Pid2(process id number of speak(2) to send a message to speak(2).
Line 9 "done" is an arbitrary atom that finishes the function start( ).
Line 10 is a call to print formated text from the input/output(io) module(library).
Line 11 starts the function speak(N).
Line 12 starts to listen for a message.
Line 13 lists the message that is received
Line 14 shows what happens when the message in 13 is received.
Line 14 prints out "hello world 1" if N is one or "hello world 2" if N is 2
Note: bang, ! in erlang means "send the following message".
