Wissenswertes über Erlang

Guido Günther

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

Extending Erlang

NIFs - Gesundheit!

Native Implemented Functions

C-Nodes

See also

Further Reading

Questions?