Erlang (programming language)

Erlang

Paradigm	multi-paradigm: concurrent, functional
Appeared in	1986
Designed by	Ericsson
Developer	Ericsson
Stable release	R14A (June 16, 2010; 7 months ago (2010-06-16))
Typing discipline	dynamic, strong
Major implementations	Erlang
Influenced by	Prolog
Influenced	Clojure, Scala
License	Modified MPL
Erlang Programming at Wikibooks

Erlang is a general-purpose concurrent, garbage-collected 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. The first version was developed by Joe Armstrong in 1986.^[1] It supports hot swapping, thus code can be changed without stopping a system.^[2] Erlang was originally a proprietary language within Ericsson, but was released as open source in 1998.

While threads are considered a complicated and error-prone topic in most languages, Erlang provides language-level features for creating and managing processes with the aim of simplifying concurrent programming. Though all concurrency is explicit in Erlang, processes communicate using message passing instead of shared variables, which removes the need for locks.

History

The name "Erlang", attributed to Bjarne Däcker, has been understood either as a reference to Danish mathematician and engineer Agner Krarup Erlang, or alternatively, as an abbreviation of "Ericsson Language".^[1][3]

Erlang was designed with the aim of improving the development of telephony applications. The initial version of Erlang was implemented in Prolog.^[1]

In 1998, the Ericsson AXD301 switch was announced, containing over a million lines of Erlang, and reported to achieve a reliability of nine "9"s. Shortly thereafter, Erlang was banned within Ericsson Radio Systems for new products, citing a preference for non-proprietary languages. The ban caused Armstrong and others to leave Ericsson.^[4] The implementation was open sourced at the end of the year.^[1] The ban at Ericsson was eventually lifted, and Armstrong was re-hired by Ericsson in 2004.^[4].

In 2006, native symmetric multiprocessing support was added to the runtime system and virtual machine.^[1]

Philosophy

The philosophy used to develop Erlang fits equally well with the development of Erlang based systems. Quoting Mike Williams, one of the three inventors of Erlang:

1. Find the right methods—Design by Prototyping.
2. It is not good enough to have ideas, you must also be able to implement them and know they work.
3. Make mistakes on a small scale, not in a production project.

Functional language

A factorial algorithm implemented in Erlang:

-module(fact).    % This is the file 'fact.erl', the module and the filename MUST match 
-export([fac/1]). % This exports the function 'fac' of arity 1 (1 parameter, no type, no name)
 
fac(0) -> 1; % If 0, then return 1, otherwise (note the semicolon ; meaning 'else')
fac(N) -> N * fac(N-1). 
% Recursively determine, then return the result 
% (note the period . meaning 'endif' or 'function end')

A sorting algorithm (similar to quicksort):

%% qsort:qsort(List)
%% Sort a list of items
-module(qsort).     % This is the file 'qsort.erl'
-export([qsort/1]). % A function 'qsort' with 1 parameter is exported (no type, no name)
 
qsort([]) -> []; % If the list [] is empty, return an empty list (nothing to sort)
qsort([Pivot|Rest]) -> 
    % Compose recursively a list with 'Front' for all elements that should be before 'Pivot'
    % then 'Pivot' then 'Back' for all elements that should be after 'Pivot'
    qsort([Front || Front <- Rest, Front < Pivot]) 
    ++ [Pivot] ++ 
    qsort([Back || Back <- Rest, Back >= Pivot]).

The above example recursively invokes the function qsort until nothing remains to be sorted. The expression [Front || Front <- Rest, Front < Pivot] is a list comprehension, meaning “Construct a list of elements Front such that Front is a member of Rest, and Front is less than Pivot.” ++ is the list concatenation operator.

A comparison function can be used for more complicated structures for the sake of readability.

The following code would sort lists according to length:

% This is file 'listsort.erl' (the compiler is made this way)
-module(listsort).
% Export 'by_length' with 1 parameter (don't care of the type and name)
-export([by_length/1]).
 
by_length(Lists) -> % Use 'qsort/2' and provides an anonymous function as a parameter
   qsort(Lists, fun(A,B) when is_list(A), is_list(B) -> length(A) < length(B) end).
 
qsort([], _)-> []; % If list is empty, return an empty list (ignore the second parameter)
qsort([Pivot|Rest], Smaller) ->
    % Partition list with 'Smaller' elements in front of 'Pivot' and not-'Smaller' elements
    % after 'Pivot' and sort the sublists.
    qsort([X || X <- Rest, Smaller(X,Pivot)], Smaller)
    ++ [Pivot] ++
    qsort([Y ||Y <- Rest, not(Smaller(Y, Pivot))], Smaller).

Here again, a Pivot is taken from the first parameter given to qsort() and the rest of Lists is named Rest. Note that the expression

[X || X <- Rest, Smaller(X,Pivot)]

is no different in form from

[Front || Front <- Rest, Front < Pivot]

(in the previous example) except for the use of a comparison function in the last part, saying “Construct a list of elements X such that X is a member of Rest, and Smaller is true", with Smaller being defined earlier as

fun(A,B) when is_list(A), is_list(B) -> length(A) < length(B) end

Note also that the anonymous function is named Smaller in the parameter list of the second definition of qsort so that it can be referenced by that name within that function. It is not named in the first definition of qsort, which deals with the base case of an empty list and thus has no need of this function, let alone a name for it.

Data structures

Erlang has eight primitive data types:

1. Integers: integers are written as sequences of decimal digits, for example, 12, 12375 and -23427 are integers. Integer arithmetic is exact and only limited by available memory on the machine.
2. Atoms: atoms are used within a program to denote distinguished values. They are written as strings of consecutive alphanumeric characters, the first character being a small letter. Atoms can obtain any character if they are enclosed within single quotes and an escape convention exists which allows any character to be used within an atom.
3. Floats: floating point numbers are represented as IEEE 754 64-bit floating point numbers. Real numbers in the range ±10³⁰⁸ can be represented by an Erlang float.
4. References: references are globally unique symbols whose only property is that they can be compared for equality. They are created by evaluating the Erlang primitive make_ref().
5. Binaries: a binary is a sequence of bytes. Binaries provide a space-efficient way of storing binary data. Erlang primitives exist for composing and decomposing binaries and for efficient input/output of binaries.
6. Pids: Pid is short for Process Identifier—a Pid is created by the Erlang primitive spawn(...) Pids are references to Erlang processes.
7. Ports: ports are used to communicate with the external world. Ports are created with the Built In Function (BIF) open_port. Messages can be sent to and received from ports, but these message must obey the so-called "port protocol."
8. Funs : Funs are function closures. Funs are created by expressions of the form: fun(...) -> ... end.

And two compound data types:

1. Tuples : tuples are containers for a fixed number of Erlang data types. The syntax {D1,D2,...,Dn} denotes a tuple whose arguments are D1, D2, ... Dn. The arguments can be primitive data types or compound data types. The elements of a tuple can be accessed in constant time.
2. Lists : lists are containers for a variable number of Erlang data types. The syntax [Dh|Dt] denotes a list whose first element is Dh, and whose remaining elements are the list Dt. The syntax [] denotes an empty list. The syntax [D1,D2,..,Dn] is short for [D1|[D2|..|[Dn|[]]]]. The first element of a list can be accessed in constant time. The first element of a list is called the head of the list. The remainder of a list when its head has been removed is called the tail of the list.

Two forms of syntactic sugar are provided:

1. Strings : strings are written as doubly quoted lists of characters, this is syntactic sugar for a list of the integer ASCII codes for the characters in the string, thus for example, the string "cat" is shorthand for [97,99,116].
2. Records : records provide a convenient way for associating a tag with each of the elements in a tuple. This allows us to refer to an element of a tuple by name and not by position. A pre-compiler takes the record definition and replaces it with the appropriate tuple reference.

Concurrency and distribution orientation

Erlang's main strength is support for concurrency. It has a small but powerful set of primitives to create processes and communicate among them. Processes are the primary means to structure an Erlang application. Erlang processes loosely follow the communicating sequential processes model (CSP). They are neither operating system processes nor operating system threads, but lightweight processes somewhat similar to Java's original “green threads”. Like operating system processes (and unlike green threads and operating system threads) they have no shared state between them. The estimated minimal overhead for each is 300 words, thus many of them can be created without degrading performance: a benchmark with 20 million processes has been successfully performed^[5]. Erlang has supported symmetric multiprocessing since release R11B of May 2006.

Process communication is done via a shared-nothing asynchronous message passing system: every process has a “mailbox”, a queue of messages that have been sent by other processes and not yet consumed. A process uses 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 and removed from the mailbox the process resumes execution. A message may comprise any Erlang structure, including primitives (integers, floats, characters, atoms), tuples, lists, and functions.

The code example below shows the built-in support for distributed processes:

 % Create a process and invoke the function web:start_server(Port, MaxConnections)
 ServerProcess = spawn(web, start_server, [Port, MaxConnections]),
 
 % Create a remote process and invoke the function
 % web:start_server(Port, MaxConnections) on machine RemoteNode
 RemoteProcess = spawn(RemoteNode, web, start_server, [Port, MaxConnections]),
 
 % Send a message to ServerProcess (asynchronously). The message consists of a tuple
 % with the atom "pause" and the number "10".
 ServerProcess ! {pause, 10},
 
 % Receive messages sent to this process
 receive
         a_message -> do_something; 
         {data, DataContent} -> handle(DataContent);
         {hello, Text} -> io:format("Got hello message: ~s", [Text]);
         {goodbye, Text} -> io:format("Got goodbye message: ~s", [Text])
 end.

As the example shows, processes may be created on remote nodes, and communication with them is transparent in the sense that communication with remote processes is done exactly as 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^[6][7]. This way of error handling increases maintainability and reduces complexity of code .

Implementation

The Ericsson Erlang implementation primarily runs interpreted virtual machine bytecode, but it also includes a native code compiler on most platforms, developed by the High Performance Erlang Project (HiPE)^[8] at Uppsala University. It also supports interpretation, directly from source code via abstract tree, via script as of R11B-5. This is also used, for example, in the Erlang shell.

Hot code loading and modules

Code is loaded and managed as "module" units; the module is a compilation unit. The system can keep two versions of a module in memory at the same time, and processes can concurrently run code from each. The versions are referred to as the "new" and the "old" version. A process will not move into the new version until it makes an external call to its module.

An example of the mechanism of hot code loading:

  %% A process whose only job is to keep a counter.
  %% First version
  -module(counter).
  -export([start/0, codeswitch/1]).
 
  start() -> loop(0).
 
  loop(Sum) ->
    receive
       {increment, Count} ->
          loop(Sum+Count);
       {counter, Pid} ->
          Pid ! {counter, Sum},
          loop(Sum);
       code_switch ->
          ?MODULE:codeswitch(Sum) 
          % Force the use of 'codeswitch/1' from the latest MODULE version
    end.
 
  codeswitch(Sum) -> loop(Sum).

For the second version, we add the possibility to reset the count to zero.

  %% Second version
  -module(counter).
  -export([start/0, codeswitch/1]).
 
  start() -> loop(0).
 
  loop(Sum) ->
    receive
       {increment, Count} ->
          loop(Sum+Count);
       reset ->
          loop(0);
       {counter, Pid} ->
          Pid ! {counter, Sum},
          loop(Sum);
       code_switch ->
          ?MODULE:codeswitch(Sum)
    end.
 
  codeswitch(Sum) -> loop(Sum).

Only when receiving a message consisting of the atom 'code_switch' will the loop execute an external call to codeswitch/1 (?MODULE is a preprocessor macro for the current module). If there is a new version of the "counter" module in memory, then its codeswitch/1 function will be called. The practice of having a specific entry-point into a new version allows the programmer to transform state to what is required in the newer version. In our example we keep the state as an integer.

In practice, systems are built up using design principles from the Open Telecom Platform which leads to more code upgradable designs. Successful hot code loading is a tricky subject; code needs to be written to make use of Erlang's facilities.

Distribution

In 1998, Ericsson released Erlang as open source to ensure its independence from a single vendor and to increase awareness of the language. Erlang, together with libraries and the real-time distributed database Mnesia, forms the Open Telecom Platform (OTP) collection of libraries. Ericsson and a few other companies offer commercial support for Erlang.

Since the open source release, Erlang has been used by several firms worldwide, including Nortel and T-Mobile.^[9] Although Erlang was designed to fill a niche and has remained an obscure language for most of its existence, its popularity is growing due to demand for concurrent services.^[10][11]

Erlang is available for many Unix-like operating systems, including Mac OS X, and for Microsoft Windows.

Projects using Erlang

Projects using Erlang include:

• RabbitMQ - an implementation of AMQP
• ejabberd - an XMPP instant messaging server
• the Facebook Chat system^[12], based on ejabberd ^[13]
• Wings 3D - a 3D modeller
• the Yaws web server
• Twitterfall - a service to view trends and patterns from Twitter^[14][15]
• Yahoo! Delicious^[16]
• CouchDB - a document based database that uses MapReduce
• Amazon SimpleDB - a distributed database that is part of Amazon Web Services^[17]
• GitHub egitd^[18] - a replacement for stock git-daemon that ships with Git (software)
• Issuu - an online digital publisher
• Scalaris - distributed key-value storage with strong consistency.
• Ringo - distributed hash table
• Disco - open source MapReduce-like framework written by Nokia
• tweet.im - XMPP gateway to Twitter written by ProcessOne
• Riak - decentralized data storage and processing system
• Chicago Boss - an MVC web framework
• Zotonic - A full fledged, fast and friendly content management system CMS (not a framework)
• Smarkets - Distributed, real-time betting exchange
• Erlyvideo - fully functional videostreaming server, that supports RTMP, RTSP, MPEG-TS, Shoutcast and streaming on IPhone.
• Heroku - A custom written load balancing system for Ruby on Rails hosting.

Clones

Erlang has inspired several clones of its concurrency facilities for other languages:

• C#: Retlang
• F#: MailboxProcessor
• JavaScript: Er.js
• Lisp: erlang-in-lisp, CL-MUPROC, Distel
• .NET: Axum an incubation project from Microsoft Research
• Perl: IPC::Messaging
• PHP: PHP/Erlang
• Python: Candygram
• Reia
• Ruby: Erlectricity
• Scala
• Scheme: Termite Scheme

See also

• Erjang

References

Further reading

External links

• Official Erlang website
• trapexit.org - Site with plenty of information about Erlang/OTP
• Erlang at the Open Directory Project
• Erlang: The Movie
• Erlang for Skeptics: A book on Erlang for beginners (work in progress)
• Learn You Some Erlang: A tutorial on Erlang for beginners
• erldocs: Alternative Erlang documentation
• tryerlang.org: An Interactive Erlang Shell in Your Browser