Erlang programming language

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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. It was designed in the company 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 is primarily interpreted, but also includes a compiler called HiPE (not supported on all platforms).

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

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 Functional language
2 Concurrency and distribution oriented language
3 Distribution
4 See also
5 External links
- 5.1 Main project
- 5.2 Other sites

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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 calls the function qsort until there is no more to be sorted. The expression [ X || X <- Rest, X < Pivot] can be read as "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.

However, a compare function can be used if the order on which Erlang bases its return value (true or false) needs to be changed. For example, if we would 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).

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Concurrency and distribution oriented language

The main strength of Erlang is support for concurrency. It has a small but powerful set of primitives to create processes and communicate between them. The process model is based on C.A.R. Hoare's Communicating Sequential Processes. Processes are the primary means to structure an application, and a very large number of them can be created without performance degradation (a benchmark with 20 millions processes was tried [1]).

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).

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
 end.

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

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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 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 became used by several companies world-wide, including Nortel and T-Mobile. However, it hasn't yet become a wide-spread mainstream programming language.

As of 2005, Erlang is under active development with regular releases. It is available for several Unix-like operating systems and Microsoft Windows.

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