---
title: "Wissenswertes über Erlang"
author: Guido Günther
date: 2016-05-09
---

Who am I
========

* Free Software Hacker (Freelancing Software Developer)
* Debian Developer since 2000
* Contributed to libvirt-*, X11, Linux Kernel, GNOME, Calypso, …
* FSFE Fellow and GNOME Foundation member
* Rather new to Erlang

Erl-what?
=========

* Developed at Ericsson in 1986
* Open sourced 1998
* Ericsson Language or Agner Krarup Erlang?
* For telephone systems

* Functional, concurrent programming language
* Garbage collected runtime system
* Erlang/OTP


Erlang, Features
================

* Fault tolerant
* Hot code replacements
* Distributed
* highly available - 12 years of uptime - nine 9s (30 ms / a)


Projects
========

* ejabberd, Riak, CouchDB, RabbitMQ
* WhatsApp, GitHub, Facebook, …


You lack style - no, state!
===========================

---

## Functional language ##

f(x) = y

---

i = i + 3

---

~~i = i + 3~~

---

for (int i=0; i < 3; i++) do_something(i);

---

~~for (int i=0; i < 3; i++) do_something(i);~~

---

    1> A=3.
    3
    2> A=4.
    ** exception error: no match of right hand side value 4

---

## No means No ##

* No imperative programming
* No global variables and state
* No pointers
* No variable reassignments

---

* Yes, Higher Order Functions
* Yes, Pattern Matching
* Yes, Code Reloads
* Yes, Lots of independent Processes

---

## Recursion to the Rescue ##

    loop(X) -> loop(X, 0).

    loop(X=0, Sum) -> Sum;
    loop(X, Sum)   -> loop(X-1, Sum+X).

---

## Recursion in Detail ##

    loop(3)   -> loop(3, 0)
    loop(3,0) -> loop(3-1, 0+3)
	loop(2,3) -> loop(2-1, 3+2)
	loop(1,5) -> loop(1-1, 5+1)
    loop(0,6) -> 6

    3 + 2 + 1 = 6

---

## Head to ~~Wall~~ Tail ##

    A = [this, is, a, list].
    [H|T] = A.

---

    rev(L) -> rev(L, []).

    rev([],    N) -> N;
    rev([H|T], N) -> rev(T, [H] ++ N).

---

    rev([a,b,c,d])       -> rev([a,b,c,d],      [])

    rev([a|[b,c,d]], []) -> rev([b,c,d], [a] ++ [])

    rev([b|[c,d]],  [a]) -> rev([c,d],  [b] ++ [a])

    rev([c|[d]],  [b,a]) -> rev([d],  [c] ++ [b,a])

    rev([d|[],  [c,b,a]) -> rev([], [d] ++ [c,b,a])

    rev([],   [d,c,b,a]) -> [d,c,b,a]

---

## More pattern matching ##

    A = {this, is, a, tuple}.
    {_, _, _, B} = A.

---

	> B.
    tuple
    > _.
    * 1: variable '_' is unbound

---

### Fun with Funs ##


    F = fun (X) -> X*X end.
    > F(3).
	> F(ok).

---

    > F(3). 
    9
    > F(ok).
    ** exception error: an error occurred when evaluating an arithmetic expression
         in operator  */2
           called as ok * ok

---

## Success, Typing! ##

Dialyzer

    -spec foobar(atom()) -> [atom()].
    foobar(App) -> …

---

Processes, which Processes
==========================

---

## Spawning a process ##


    F = fun(X) -> io:format("~p~n", [X]) end.
    times(X, Args, C) -> ...

    spawn(l, times, [F,  "I'm a process", 3]).
    

---


    spawn(l, times, [F,  "I'm a process", 3]),
    spawn(l, times, [F, "I'm a process too", 3]).

---


## Concurrency for free! ##

	"I'm a process"
	"I'm a process too"
	"I'm a process"
	"I'm a process too"
	<0.53.0>
	"I'm a process"
	"I'm a process too"


---

## Share(d) nothing ##


    start_pong(Pid) -> …
    pong(Pid) -> …
	
	> P = l:start_pong(self()).
    > P ! { ping, "foo" }.
	> P ! giveup.


---

    > R = fun () -> receive M -> M end end.

---

    > R().
    {pong,"hallo1"}
    > R().
    {pong,"hallo2"}

---

## Let it crash ##

	2> P ! {ping, 3}.
	{ping,3}
	3> 
	=ERROR REPORT==== 8-May-2016::15:13:30 ===
	Error in process <0.34.0> with exit value:
	{badarg,[{io,format,[<0.25.0>,"pong: Received ping '~s'~n",[3]],[]},
			 {l,pong,1,[{file,"l.erl"},{line,57}]}]}

---

	2> P ! whatever
    Unknwown message asfasdf, giving up

---

## Linking processes ##

    > process_flag(trap_exit, true).
    > P = l:start_pong2(self()).
	3> P ! sadfsadf.
    Unknwown message sadfsadf, giving upsadfsadf
    4> receive X -> X end.
    {'EXIT',<0.35.0>,reason}

---

## Distributed nodes ##

    $ erl -sname ping
    $ erl -sname pong
	
	register(spawn(...))

---

On pong

    > P = l:start_pong3().

On ping

    > {pong, pong@bogon} ! {ping, "hello", self()}.
    > {pong, pong@bogon} ! {ping, "hello", self()}.
    > receive X -> X end.
    {pong,"hello"}

---

    erlang:node().
    erlang:nodes().

---

## Hot code replacements


    version() -> …
    c(l).

---

OTP
===

Open Telecommunication Platform

---

## Batteries inclued ##

* EUnit
* Mnesia
* HTTP/SNMP/…
* …

---

## Behaviours ##

* gen_{server,fsm,event}
* supervisor
* application

---

Debugging
=========

---

## Tracing Processes, Messges, … ##


    erl -sname observer -hidden	 -run observer

---

## Tracing functions ##

	dbg:start().
	dbg:tracer().
	% trace messages (m)
	dbg:p(pong, m).	 

---

The greater Erlang Universe
===========================

* REST: [Webmachine](https://github.com/webmachine/webmachine)
* JSON: [jiffy](https://github.com/davisp/jiffy)
* YAML: [p1_yaml](https://github.com/processone/p1_yaml)
* Build, run, test: [Rebar3](https://github.com/erlang/rebar3)
* … your favorite erlang app goes here …

---

Extending Erlang
================

---

## NIFs - Gesundheit! ##

Native Implemented Functions

---

## C-Nodes

See also
========

* [Elixir](http://elixir-lang.org/)
* [LFE(Lisp flavored Erlang)](http://lfe.io/)
* [Erlang on Xen](http://erlangonxen.org/)

Further Reading
===============

* [Learn you some erlang](http://learnyousomeerlang.com/) - buy the book!
* On Debian [/usr/share/doc/erlang-doc]()
* Everywhere else: https://www.erlang.org/docs

Questions?
==========