An introduction to Erlang I gave to the Kansas City Ruby Users Group on July 14, 2009.

1. Erlang/OTP
for Rubyists
Sean Cribbs
KC.rb - July 2009

2. Ruby
Makes easy things trivial
and hard things fun!

3. Erlang
Makes easy things possible
and impossible things trivial!
Orion Henry and Blake Mizerany
(Heroku)

4. What is Erlang?
•Language, Runtime and Libraries (OTP)
•Developed and maintained by Ericsson
•Started mid 1980s, open-sourced 1998

5. Who uses it?

6. Who uses it?

7. Object-oriented
Programming

8. object object object
object object
object
object object
object object
object object
Object-oriented
object Programming object
object object
object object
object object
object
object

9. Object-oriented
Programming

10. Concurrency-oriented
Programming

11. process process
process process
process
process process
process process
process
process process
Concurrency-oriented
process
Programming process
process process
process
process process
process
process process

12. Concurrency-oriented
Programming

13. Fault-tolerance
IFF
Concurrency

14. Error Handling
Intra-process
Inter-process
Inter-machine

15. Sharing doesn’t scale
•Deadlocks
•Race Conditions
•Starvation
•Synchronization
•Data Locality
•Corruption

16. Message passing
is the answer

17. Cool Features
•Concurrent
•Distributed
•Share-nothing
•Fault-tolerant
•Hot upgrades
•Highly available

18. Big Differences
•Functional - no objects
•Single assignment, few side-effects
•Initially challenging syntax
•Nothing like gems

19. Are you ready to be
UNCOMFORTABLY
PARALLEL?

20. Spawn
Pid = spawn(
fun()->
io:format("Hello, Joe.")
end).

21. Send Messages
Pid ! {greet, “Joe”}.

22. Receive Messages
loop() ->
receive
{greet,S} ->
io:format(“Hello, ~s”, [S]),
loop();
stop -> ok;
_ -> loop()
end.

23. Syntax Overview

24. Numbers
1
-1
2501
3.14
12345678987654321.987654321

25. Atoms
foo
type
true
false
‘EXIT’

26. Lists
[1,2,3]
[“dog”, “cat”, “fish”]
“hello, world”

27. Tuples
{name, “Sean”}
{address, “8700 State Line Rd”, “Leawood”,
“KS”}
{{line, 10},{column,30}}
{{2009,7,14},{19,0,0}}

28. Binaries
<<“chunk of bytes”>>
<<131,108,0,0,0,2,100,0,3,111,110,101,100
,0,3,116,119,111,
106>>

29. Some Weirdos
“abc” == [97,98,99]. % lists of bytes
true, false % atoms
$a == 97. % characters (ASCII)
P#person.name % compile-time struct

30. Assignment

31. Pattern Matching

32. Pattern Matching
A = 5.
B = 10.
Group = “KC.rb”.

33. Pattern Matching
{Type, Value} = {username, “sean”}.
[{username, Value},
{orders, Orders}] = [{username, “sean”},
{orders, [1,2,3]}].

34. Single Assignment
Really, this is just pattern matching.
A = 5.
A = A + 1. % error: badmatch (5 = 6)

35. Lists
functional programming’s old friend

36. List Operations
[First|Rest] = [1,2,3,4,5].
[H|T] = [1,2,3,4,5].
List1 ++ List2.

37. List Comprehensions
Think select/filter + map.
A = [1,2,3,4,5].
[X * 2 || X <- A]. % [2,4,6,8,10]
[X || X <- A, X rem 2 == 0]. % [2,4]

38. Functions
even_or_odd(Num) ->
if
Num rem 2 == 0 ->
even;
true ->
odd
end.

39. Functions
even_or_odd(N) when N rem 2 == 0 ->
even;
even_or_odd(N) ->
odd.

40. Functions
even_or_odd(N) when is_number(N),
N rem 2 == 0 ->
even;
even_or_odd(N) when is_number(N) ->
odd.

41. Anonymous Functions
like lambda/proc, block
Double = fun(X) -> 2*X end.
Double(2). % 4
lists:map(Double, [1,2,3,4,5]).
% [2,4,6,8,10]

42. No Loops!
use recursion

43. A Little CS Theorem
iterative === recursive

44. List “iteration”
print([]) ->
ok;
print([H|T]) ->
io:format(“~p”, [H]),
print(T).

45. List “iteration”
print([]) ->
ok;
print([H|T]) ->
io:format(“~p”, [H]),
print(T).
print(T).

46. Normal Recursion
loop()
loop()
loop()
loop()

47. Tail Recursion
(aka tail-call optimization)
loop()

48. Concurrent programs
can be complicated.

49. Behaviors
“patterns”
•gen_server - client-server
•gen_fsm - state machine
•supervisor - fault-tolerance

50. How does it scale?
•Micro-benchmarks, arithmetic slow
•Best for network(ed) apps
•Campfire just as fast as C, ~ 2-4ms
•Near-linear speedup on multicores
•GC per-process, generational

51. Sean ! Questions.