Hidden Features of Erlang

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.

Solution 1

The magic commands in the shell. The full list is in the manual, but the ones I use most are:

• f() - forget all variables
• f(X) - forget X
• v(42) - recall result from line 42
• v(-1) - recall result from previous line
• e(-1) - reexecute expression on previous line
• rr(foo) - read record definitions from module foo
• rr("*/*") - read record definitions from every module in every subdirectory
• rp(expression) - print full expression with record formating

Solution 2

Inheritance! http://www.erlang.se/euc/07/papers/1700Carlsson.pdf

Parent

-module(parent).
-export([foo/0, bar/0]).

foo() ->
    io:format("parent:foo/0 ~n", []).

bar() ->
    io:format("parent:bar/0 ~n", []).

Child

-module(child).
-extends(parent).
-export([foo/0]).

foo() ->
    io:format("child:foo/0 ~n", []).

Console

23> parent:foo().
parent:foo/0 
ok
24> parent:bar().
parent:bar/0 
ok
25> child:foo().
child:foo/0 
ok
26> child:bar().
parent:bar/0 
ok

Solution 3

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

Solution 4

user_default.erl - you can build your own shell builtins by having a compiled user_default.beam in your path which can be pretty nifty

Solution 5

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))]).

View more solutions

Comments

[Ólafur Waage]

Please Community Wiki this.

[ryeguy]

How is this a hidden feature? A notable feature for sure, but this is one of the language's primary boasting points, not hidden at all.

[John]

I spotted this when I was trying to grok the mochiweb source. Took a while to google what the hell it was actually doing, as the syntax was totally different to the Erlang I've seen before.

[Christian]

And using rp(expression(...)) to have the result printed out without pretty-printing too deeply nested structures, instead it prints it fully

[Paul Ankman]

Didn't know that, very useful, thanks!

[bjnortier]

Yup. Interesting! Although I haven't seen it in any libraries I've used to date...

[YasirA]

However it's not welcomed to use parameterized modules: They add variables to functions and it's not good when it comes to debug them. They break the functional paradigm adding a layer of complexity introduced by variables you should keep track of (like global variables in imperative languages), they are immutable but destroy transparency. There've been the thoughts that they (parameterized modules) still exist in the language only because of products utilizing them, mochiweb is an example. Just think, why appeared in 2003 they still officially undocumented?

[BalusC]

For the case you wonders the downvotes, this was an accident, please check this meta topic. I'll edit your question so that they can be removed.

[bjnortier]

This comment belongs to the "Parameterized Modules" answer...

[Anton Kraievyi]

But why the hell no upvotes? :D