Hidden Features of Erlang [closed]

Question

In the spirit of:

• Hidden Features of C#
• Hidden Features of Java
• Hidden Features of ASP.NET
• Hidden Features of Python
• Hidden Features of HTML
• and other Hidden Features questions

What are the hidden features of Erlang that every Erlang developer should be aware of?

One hidden feature per answer, please.

Answer 1

That match specifications can be built using ets:fun2ms(...) where the Erlang fun syntax is used and translated into a match specification with a parse transform.

1> ets:fun2ms(fun({Foo, _, Bar}) when Foo > 0 -> {Foo, Bar} end).
[{{'$1','_','$2'},[{'>','$1',0}],[{{'$1','$2'}}]}]

So no fun-value is ever built, the expression gets replaced with the match-spec at compile-time. The fun may only do things a match expression could do.

Also, ets:fun2ms is available for usage in the shell, so fun-expressions can be tested easily.

Answer 2

Parameterized Modules! From http://www.lshift.net/blog/2008/05/18/late-binding-with-erlang and http://www.erlang.se/euc/07/papers/1700Carlsson.pdf

-module(myclass, [Instvar1, Instvar2]).
-export([getInstvar1/0, getInstvar2/0]).
getInstvar1() -> Instvar1.
getInstvar2() -> Instvar2.

And

Eshell V5.6  (abort with ^G)
1> Handle = myclass:new(123, 234).
{myclass,123,234}
2> Handle:getInstvar1().
123
3> Handle:getInstvar2().
234

Answer 3

beam_lib:chunks can get source code from a beam that was compiled with debug on which can be really usefull

{ok,{_,[{abstract_code,{_,AC}}]}} = beam_lib:chunks(Beam,[abstract_code]).
  io:fwrite("~s~n", [erl_prettypr:format(erl_syntax:form_list(AC))]).

Answer 4

The gen___tcp and ssl sockets have a {packet, Type} socket option to aid in decoding a number of protocols. The function erlang:decode_packet/3 has a good description on what the various Type values can be and what they do.

Together with a {active, once} or {active, true} setting, each framed value will be delivered as a single message.

Examples: the packet http mode is used heavily for iserve and the packet fcgi mode for ifastcgi. I can imagine that many of the other http servers use packet http as well.

Answer 5

Hot code loading. From wiki.

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 the "new" and the "old" version. A process will not move into the new version until it makes an external call to its module.

Answer 6

.erlang_hosts gives a nice way to share names across machines