Erlang (programming language)

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**Erlang**
Paradigm:	multi-paradigm: concurrent, functional
Appeared in:	1987
Designed by:	Ericsson
Developer:	Ericsson
Typing discipline:	dynamic, strong
Major implementations:	Erlang

Erlang is a general-purpose concurrent programming language and runtime system. The sequential subset of Erlang is a functional language, with strict evaluation, single assignment, and dynamic typing. For concurrency it follows the Actor model. It was designed by Ericsson to support distributed, fault-tolerant, soft-real-time, non-stop applications. It supports hot swapping so code can be changed without stopping a system. Erlang was originally a proprietary language within Ericsson, but was released as open source in 1998. The Ericsson implementation primarily runs interpreted virtual machine code, but it also includes a native code compiler (not supported on all platforms), developed by the High-Performance Erlang Project (HiPE) at Uppsala University. It also now supports interpretation via escript as of r11b-4.

Creating and managing processes is trivial in Erlang, whereas threads are considered a complicated and error prone topic in most languages. In Erlang, all concurrency is explicit.

Erlang is named after A. K. Erlang. It is sometimes thought that its name is an abbreviation of ERicsson LANGuage, owing to its heavy use inside Ericsson. According to Bjarne Däcker, who headed the Computer Science Lab at the time, this duality is intentional.[1]

1 Functional language
2 Concurrency and distribution oriented language
3 Distribution
4 See also
5 Notes
6 Further reading
7 External links

[edit] Functional language

Code looks like this:

-module(fact).
-export([fac/1]).

fac(0) -> 1;
fac(N) when N > 0 -> N * fac(N-1).

Below is an implementation of a Quicksort algorithm.

%% quicksort:qsort(List)
%% Sort a list of items
-module(quicksort).
-export([qsort/1]).

qsort([]) -> [];
qsort([Pivot|Rest]) ->
    qsort([ X || X <- Rest, X < Pivot]) ++ [Pivot] ++ qsort([ Y || Y <- Rest, Y >= Pivot]).

The above example recursively invokes the function qsort until nothing remains to be sorted. The expression [ X || X <- Rest, X < Pivot] means "Choose all X where X is a member of Rest and X is less than Pivot", resulting in a very easy way of handling lists. Since you can evaluate any boolean expression between two different datatypes, the evaluation is straightforward: for example, 1 < a will return true.

A compare function can be used, however, if the order on which Erlang bases its return value (true or false) needs to be changed. If, for example, we want an ordered list where a < 1 evaluates true.

The following code would sort lists according to length:

-module(listsort).
-export([by_length/1]).

by_length(Lists) ->
    F = fun(A,B) when is_list(A), is_list(B) ->
            length(A) < length(B)
        end,
    qsort(Lists, F).

 qsort([], _) -> [];
 qsort([Pivot|Rest], Smaller) ->
     qsort([ X || X <- Rest, Smaller(X, Pivot)], Smaller)
     ++ [Pivot] ++
     qsort([ Y || Y <- Rest, not(Smaller(Y, Pivot))], Smaller).

[edit] Concurrency and distribution oriented language

Erlang's main strength is support for concurrency. It has a small but powerful set of primitives to create processes and communicate between them. Processes are the primary means to structure an Erlang application. Erlang processes are neither OS processes nor OS threads, but lightweight threads somewhat similar to Java's "green threads". They are, as a result, extremely lightweight (the estimated minimal overhead for each is 300 bytes) and many of them can be created without degrading performance (a benchmark with 20 million processes was tried^[1]). Erlang has supported symmetric multiprocessing since release R11B of May 2006.

The processes communicate via a share-nothing asynchronous message-passing system: every process has a "mailbox", a queue of messages sent by other processes, that are not yet consumed. Threads then use the receive primitive to retrieve messages that match desired patterns. A message-handling routine tests messages in turn against each pattern, until one of them matches. When the message is consumed (removed from the mailbox) the thread resumes execution. A message may comprise any Erlang structure, including primitives (integers, floats, characters, atoms), tuples, lists, and functions.

Code examples:

 Pid = spawn(Mod, Func, Args)       % execute function Func as new process
 Pid = spawn(Node, Mod, Func, Args) % execute function Func in remote node Node

 Pid ! a_message      % send message to the process (asynchronously)

 receive       % receive message sent to this process
   a_message -> do_something;
   {data, Data_content} -> do_something_else(); % This is a tuple of a type atom and some data
   {hello, Text} -> io:format("Got hello message: ~s", [Text]);
   {goodbye, Text} -> io:format("Got goodbye message: ~s", [Text])
 end.

There is also built-in support for distributed processes. Processes may be created on remote nodes, and communication with them is transparent (i.e. the communication with remote processes is done exactly as the communication with local processes).

Concurrency supports the primary method of error-handling in Erlang. When a process crashes, it neatly exits and sends a message to the controlling process which can take action. This way of error handling may increase maintainability and reduce complexity of code.

[edit] Distribution

Erlang was released by Ericsson as open-source to ensure its independence from a single vendor and to increase awareness of the language. Distribution of the language together with libraries and a real-time distributed database (Mnesia) is known as the Open Telecom Platform, or OTP. Ericsson and a few other companies offer commercial support for Erlang.

Since taking the open-source path in 1998 it has been used by several companies world-wide, including Nortel and T-Mobile^[2], although it has not become a widespread programming language.

As of 2006, Erlang is under active development with regular releases. It is available for several Unix-like operating systems and Microsoft Windows.

[edit] See also

ejabberd, an XMPP/Jabber instant messaging server written in Erlang.
Wings 3D, 3D modeller written in Erlang.
Yaws (Yet another web server) a web server written in Erlang.
Tsung, a high-performance benchmarking tool.
ErlLounge, Gatherings where Erlang is discussed.

[edit] Notes

^ Ulf Wiger (2005-11-14). Stress-testing erlang. comp.lang.functional.misc. Retrieved on August 25, 2006.
^ Who uses Erlang for product development?. Frequently asked questions about Erlang. Retrieved on June 27, 2006. "The largest user of Erlang is (surprise!) Ericsson. Ericsson use it to write software used in telecommunications systems. Many (dozens) projects have used it, a particularly large one is the extremely scalable AXD301 ATM switch." Other commercial users listed as part of the FAQ include: Nortel, Deutsche Flugsicherung (the German national air traffic control organisation), and T-Mobile

[edit] Further reading

Joe Armstrong (2003). "Making reliable distributed systems in the presence of software errors". Ph.D. Dissertation. The Royal Institute of Technology, Stockholm, Sweden.
Joe Armstrong (2007). "Programming Erlang, Software for a Concurrent World". Pragmatic Programmers.
Mattsson, H., Nilsson, H., Wikstrom, C.: "Mnesia - A distributed robust DBMS for telecommunications applications." First International Workshop on Practical Aspects of Declarative Languages (PADL'99) (1999) pages 152-163
Erlang Style Concurrency