Erlang (programming language)

The Erlang programming language is a strict functional language, a declarative language, and a general-purpose programming language which shares some syntax with prolog. It is considered declarative because it has the pattern matching syntax of prolog and list comprehensions. Because of its prowess at parallel programming it is particulary good at creating servers such as web servers or ftp servers with small amounts of code. It is a dynamically typed language. Armstrong also describes it as a concurrency oriented language. Erlang was developed in 1987 by Joe Armstrong and others (then of Ericsson) for use to program telephone networks. Ref: Joe Armstrong (2003). "Making reliable distributed systems in the presence of software errors". Ph.D. Dissertation. New versions of Erlang are released by Ericsson on a yearly basis. At present(2008) there is increased interest in parallel programming languages because of the use of multicore microprocessors in personal computers.

Syntax

Hello World

-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.

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".

See also

• Prolog
• Scheme
• Erlang Tutorials