Erlang (programming language)/Tutorials

Erlang Language Programming Tutorials

Examples

Hello World (serial)

Code Example

-module(hello).
-export([start/0]).

start() ->
   io:format("Hello, world!\n").

Analysis of the example

The Hello World program (see above) appears in many programming languages books and articles as a cursory introduction into a language's syntax. The first hello world program was introduced in the book The C Programming Language.

-module(hello) tells the compiler to create a new module(library) called hello. The code tells us the file name for this code: hello.erl.

-export([start/0]). exports a function named start with 0 arguments to the world outside of this module called hello.

start() -> tells the compiler that there is a function named start() with no arguments.

io:format("Hello, world!\n"). will make the program output Hello, world! and a new line (\n) on the screen.

Hello World (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".

Prime Sieve (parallel with linda type coordination)

How many processes can this program use? This program creates as many sieves as the square root of the numbers in the matrix. If we are looking for the primes below 100 then there are ~10 parallel sieve processes. Actually, most of the seive processes are halted and only (the number of prime numbers under the square root of Max) processes are left at the end. This allows an easy parallelism of 10 for 100 and 100 for 10000 with little modification.

Prime Sieve Program (parallel)

   -module(primes).

   % This is a simple linda tuplespace. Here we use it to find primes numbers.
   % This tuple-space can not have duplicate tuples, but with a prime sieve it does
   %  not matter.

   -compile(export_all).

   start() -> start(100).  % defualt value for max is 100
   start(Max) -> 
       io:format("  Loading ~w numbers into matrix (+N) \n ", [ Max ] ),
       Lid = spawn_link( primes, linda, [Max, [], [] ]),
       Sqrt = round(math:sqrt(Max)+0.5),  
       io:format(" Sqrt(~w) + 1 = ~w \n " , [Max,Sqrt] ),  
       io:format(" Tuple space is started ~n ",[]),  
       io:format(" ~w sieves are spawning (+PN) ~n ", [Sqrt] ),
       io:format(" Non prime sieves are being halted (-PN) ~n ", [] ),
       % load numbers into tuplespace 
       % and spawn seive process
       spawn( primes, put_it, [Max, Max, Lid] ).

   start_sieves(Lid) ->
       Lid ! {self(), get, all, pids},  
       receive 
           {lindagram, pids, Pids} -> done
       end,
       start_sieve_loop(Pids).

   start_sieve_loop([]) -> done;
   start_sieve_loop([Pid|Pids]) ->
       receive
       after 100 -> done
       end,
       Pid ! {start},
       start_sieve_loop(Pids).

   spawn_sieves( _Max, Sqrt, _Lid, Sqrt ) -> done;  
   spawn_sieves( Max, Inc, Lid, Sqrt ) ->
       T = 1000,
       Pid = spawn( primes, sieve, [ Max, Inc+Inc, Inc, Lid, T ]),
       Name = list_to_atom("P" ++ integer_to_list(Inc)),
       Lid ! {put, pid, Name},
       register( Name, Pid),
       io:format(" +~s ", [atom_to_list(Name)]),
       spawn_sieves( Max, Inc+1, Lid, Sqrt ).

   put_it(Max, N, Lid) when N =< 1 ->
       Sqrt = round(math:sqrt(Max)+0.5),
       spawn_sieves( Max, 2, Lid, Sqrt );  

   put_it(Max, N,Lid) when N > 1 ->
       receive
       after 0 ->
           Lid ! {put, N, N},
           if 
               N rem 1000 == 0 ->
                   io:format(" +~w ", [N]);
               true -> done
           end,
           put_it(Max, N-1,Lid)
       end.

   % the 2 sieve starts last and has the most to do so it finishes last
   sieve(Max, N, 2, Lid, _T) when N > Max -> 
       io:format("final sieve ~w done, ~n", [2] ),
       Lid ! {dump,output};

   sieve(Max, N, Inc, _Lid, _T) when N > Max ->    
       io:format("sieve ~w done ", [Inc] );

   sieve(Max, N, Inc, Lid, T) when N =< Max ->   
       receive 
       after 
           T -> NT = 0   
       end,
       receive 
           {lindagram,Number} when Number =/= undefined -> 
               clearing_the_queue;
           {exit} -> exit(normal)
       after
           1 -> done 
       end,

       % remove multiple of number from tuple-space
       Lid ! {self(), get, N},
       Some_time = (N rem 1000)==0,
       if Some_time -> io:format("."); true -> done end,

       % remove (multiple of Inc) from sieve-process space
       Name = list_to_atom("P" ++ integer_to_list(N)),
       Exists = lists:member( Name, registered()),
       if 
           Exists ->
               Name ! {exit},
               io:format(" -~s ", [atom_to_list(Name)] );
           true -> done
       end,
       sieve(Max, N+Inc, Inc, Lid, NT).        % next multiple
       
   %% linda is a simple tutple space 
   %%    if it receives no messages for 2 whole seconds it dumps its contents 
   %%    as output and halts

   linda(Max, Keys, Pids) ->
       receive
       {put, pid, Pid} ->
           linda(Max, Keys, Pids++[Pid]);
       {put, Name, Value} ->
           put( Name, Value),
           linda(Max, Keys++[Name], Pids);
       {From, get, Name} ->
           From ! {lindagram, get( Name)},
           erase( Name ),                          % get is a desructive read  
           linda(Max, Keys--[Name],Pids);
       {From, get, all, pids} ->
           From ! {lindagram, pids, Pids},
           linda(Max, Keys, Pids );
       {From, get, pid, Pid} ->
           L1 = length( Pids ),
           L2 = length( Pids -- [Pid]),
           if 
               L1 > L2 ->  % if it exists
                   From ! {lindagram, pid, Pid};
               true -> 
                   From ! {lindagram, pid, undefined}
           end,
           linda(Max, Keys, Pids );
       {dump,output} ->
           io:format(" ~w final primes remain: ~w ~n ", [length(Keys),  lists:sort(Keys) ])
       after (100*Max) -> % if there is not tuple action after some time then print the results
           io:format(" ~w primes remain: ~w ~n ", [length(Keys),  lists:sort(Keys) ])
       end.

Sample Output for Prime Sieve

c(primes).
primes:start(1000).
 Loading 1000 numbers into matrix (+N)
 Sqrt(1000) + 1 = 32
 Tuple space is started
 32 sieves are spawning (+PN)
 Non prime sieves are being halted (-PN)
 +1000 <0.46.0>
+P2  +P3  +P4  +P5  +P6  +P7  +P8  +P9  +P10  
+P11  +P12  +P13  +P14  +P15  +P16   
+P17  +P18  +P19  +P20  +P21  +P22  +P23  +P24  
+P25  +P26  +P27  +P28  +P29  +P30  
+P31  -P8  -P6  -P4  -P9  -P12  -P10  -P15  
-P15  -P18  -P14  -P21  -P21  -P22  
-P26  -P20  -P24  -P25  -P27  -P28  -P30  -P30  -P16 
sieve 31 done sieve 29 done 
sieve 19 done sieve 23 done sieve 11 done 
sieve 13 done sieve 17 done sieve 7 done 
.sieve 5 done sieve 3 done .final sieve 2 done,
168 final primes remain: 
[2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,
71,73,79,83,89,97,
101,103,107,109,113,127,131,137,139,149,151,157,163,
167,173,179,181,191,193,197,199,
211,223,227,229,233,239,241,251,257,263,269,271,277,281,283,293,
307,311,313,317,331,337,347,349,353,359,367,373,379,383,389,397,
401,409,419,421,431,433,439,443,449,457,461,463,467,479,487,491,
499,503,509,521,523,541,547,557,563,569,571,577,587,593,599,
601,607,613,617,619,631,641,643,647,653,659,661,673,677,683,691,
701,709,719,727,733,739,743,751,757,761,769,773,787,797, 
809,811,821,823,827,829,839,853,857,859,863,877,881,883,887,
907,911,919,929,937,941,947,953,967,971,977,983,991,997]

Autonomous Agents

See definition of Autonomous Agent.

Dynamic timeout based initiative switching

Explanation of the code in jungle.erl

Here we have a simple chat-bot agent called person/4. We create two instances of it called Tarzan and Jane. They talk to each other. Each has a timeout. The timeout is the lenght of time one will wait before they intiate conversation. The initial timeout of Jane is set to 10 seconds. The initial timeout of Tarzan is set to 8 seconds. Because of the initial values, Tarzan will speak first and Jane will respond. Both timeouts start over but keep the same values. Again, Tarzan speaks first and Jane responds. Now things get interesting. The agent can tell if the conversation is repeating. If the conversation repeats then special messages are sent to cause a swap in the relative levels of timeout. Now Tarzan waits longer than Jane and Jane has a chance to speak first. Now, Jane speaks first twice. Then they swap initiative again. Since the processes are autonomous, we need to stop them with a quit program called jungle:quit().

Example program listing: jungle.erl

-module( jungle ).
-compile(export_all).
   
%% This program shows how chat-bot agents can exchange initiative(lead) while in conversation.
%%   Start with start().
%%   End with quit().
 
start() ->
    register( tarzan, spawn( jungle, person, [ tarzan,  8000, "", jane ]   ) ),
    register( jane, spawn( jungle, person,   [ jane,   10000, "", tarzan ] ) ),
    "Dialog will start in 5ish seconds, stop program with jungle:quit().".
                                                                                         
quit() ->
    jane ! exit,
    tarzan ! exit.
 
%% Args for person/4
%%   Name:  name of agent being created/called
%%   T:     timeout to continue conversation
%%   Last:  Last thing said
%%   Other: name of other agent in conversation
  
person( Name, T, Last, Other ) ->
    receive
        "hi" -> 
            respond( Name, Other, "hi there \n " ),
            person( Name, T, "", Other );
        "slower" -> 
            show( Name, "i was told to wait more " ++ integer_to_list(round(T*2/1000))),
            person( Name, T*2, "", Other );
        "faster" -> 
            NT = round( T/2 ),
            show( Name, "I was told to wait less " ++ integer_to_list(round(NT/1000))),
            person( Name, NT, "", Other );
        exit ->
            exit(normal);
        _AnyWord -> 
            otherwise_empty_the_queue,
            person( Name, T, Last, Other )
    after T -> 
       respond( Name, Other, "hi"),
       case Last of
           "hi" ->
           self() ! "slower",
           sleep( 2000),                  % give the other time to print
           Other ! "faster",
           person( Name, T, "", Other );
       _AnyWord ->
           person( Name, T, "hi", Other )
       end
   end.
                                                                                             %
respond( Name, Other, String ) ->
    show( Name, String ),
    Other ! String.
                                                                                             %
show( Name, String ) ->
    sleep(1000),
    io:format( " ~s -- ~s \n ", [ Name, String ] ).
                                                                                             %
sleep(T) ->
    receive 
    after T ->
        done
    end.
% ===========================================================>%

Sample output from: jungle.erl

Sample output:
 	
 18> c(jungle).
 {ok,jungle}
  
 19> jungle:start().
 jane_and_tarzan_will_start_in_5_seconds
  
 tarzan -- hi 
 jane -- hi there 
 
 tarzan -- hi 
 jane -- hi there 
 
 jane -- I was told to wait less: 5 
 tarzan -- I was told to wait more: 16 
  
 jane -- hi 
 tarzan -- hi there 
 
 jane -- hi 
 tarzan -- hi there 
 
 tarzan -- I was told to wait less: 8 
 jane -- I was told to wait more: 10 
 
 tarzan -- hi 
 jane -- hi there 
 
 tarzan -- hi 
 jane -- hi there 
 
 jane -- I was told to wait less: 5 
 tarzan -- I was told to wait more: 16 
 
 jane -- hi 
 tarzan -- hi there 
 
20> jungle:quit(). 
exit

References

1 - Erlang Man Pages at Erlang,org