1. Erlang http://erlang.org/ Noah Dietrich Chris Ruffalo

2. 2 Introduction to Erlang Designed by Ericson for phone switches. Released Open Source in 1998. Interpreted VM code.

3. 3 Strengths & Uses Distributed, fault-tolerant, soft-real-time. Hot-swap code Lightweight threading, SMP.

4. 4 Using Erlang Open Source – Download from http://erlang.org Read-Eval loop Run Erlang from your source directory:

5. 5 Variables: Numbers Terms Integer Float

6. 6 Examples: Numbers >1 + 1. 2 >1000000000000000000000 + 3. 1000000000000000000003 >.1 +2. 2.1

7. 7 Variables: Continued Atoms Tuples Lists Bit String

8. 8 Examples Example 1: > myAtom. myAtom > ‘this is also an atom’. 'this is also an atom' Example 2: >X = {person, 'bob smith'}. {person, 'bob smith'} > {person, Who} = X. {person, 'bob smith'} >Who. 'bob smith' Example 3: > Y = {person, {name, 'bob smith'}, {age, 22} }. {person, {name, 'bob smith'}, {age, 22} } > {_, {_, _}, {_, ClientAge } = Y. {person, {name, 'bob smith'}, {age, 22} } > ClientAge. 22

9. 9 Working With Variables Single Assignment – one value for lifetime of variable. Pattern matching – Equals sign is not assignment, but pattern match. Infix Notation Short-circuit Evaluation Type conversion

10. 10 Examples Short-Circuit Expressions –Expression_1 orelse Expression_2 –Expression_1 andalso Expression_2 Type Conversions With BIF's > atom_to_list(hello). "hello" > list_to_atom("hello"). hello

11. 11 Advanced Types PID (process ID) Port Reference Fun (anonymous function)

12. 12 Not Types Boolean Char String Record

13. 13 Examples Lists: "hello" is shorthand for the list [$h,$e,$l,$l,$o], which is really: [104,101,108,108,111]. Boolean: 1> 2 =< 3. true 2> true or false. true

14. 14 Control Statements If Case Guards No For-loop

15. 15 Guards Used in conditional statements. Guard Expression: Guard Guard Sequence

16. 16 Guard Examples Guard Expressions: X > 2 Y = 4 Guard (true if ALL guard expressions are true): X > 2, Y = 4, N >= 2 Guard Sequence (true if One guard is true) X > 2, Y = 4, N >= 2 ; is_float(W), W > 0

17. 17 If Expression if GuardSeq1 -> Body1;...; GuardSeqN -> BodyN; true -> % works as an else branch end.

18. 18 Case Expression case Expr of Pattern1 [when GuardSeq1] -> Body1;...; PatternN [when GuardSeqN] -> BodyN _Else -> BodyElse; end.

19. 19 For Loop Erlang does not support looping Use recursive function calls instead.

20. 20 Memory Management Garbage Collection Threads and Memory Overhead Activation Records

21. 21 Modules Contain attributes and functions Expose interface Compiled into.beam files (object code) Compiled & loaded from interpreter with –> c(Module)

22. 22 Module Example -module(m). -export([fact/1]). fact(N) when N>0 -> N * fact(N-1); fact(0) -> 1. Save this code in a file called m.erl Compile in Erlang shell with: c(m). c stands for compile. Make sure to run Erlang interpreter from the same directory as where your code is saved. Run factorial in Erlang shell by calling: m.fact(20).

23. 23 Functions Pattern match implementation First-class functions Tail Recursion Use Guard Sequences to identify correct clause

24. 24 Function Example -module(euler). -export([compute/1]). % Find the sum of all the multiples of 3 or 5 below 1000. compute(N) when N >= 1000 -> 0; compute(N) when N rem 5 =:= 0;N rem 3 =:= 0 -> N + compute (N+1) compute(N) -> compute (N + 1).

25. 25 Functional Object (Anonymous Functions) 1> Fun1 = fun (X) -> X+1 end. #Fun 2> Fun1(2). 3

26. 26 List Operations Erlang has many methods for operating on or across lists. –Maps –For Each –Folding –Comprehensions

27. 27 Mapping Erlang allows developers to use a special built in function in the lists module to apply a function across an entire list. Can be used with normally declared functions or with anonymous functions.

28. 28 Mapping Example Code: raise_list(List) -> lists:map(fun pow/1,List). pow({X,Y}) -> math:pow(X,Y). Input: raise_list([{1,2},{3,4},{4,5}]). Output: [1.0,81.0,1024.0]

29. 29 For Each The foreach function in the lists module is similar to map, except that no value is returned. Good for things like sending messages to threads or sockets where no (direct) return is expected.

30. 30 Folding The foldl and foldr functions in the lists module can be used for situations where an accumulator must be used instead of a list return. Sum, average, counts, etc. The l and r that proceed the fold indicate the directionality. The foldl function is preferred over foldr because it is tail recursive.

31. 31 Folding Example Code: sum(List) -> lists:foldl( fun (Element, Acc) -> Acc + Element end, 0, List ). Input: sum([1,2,3]) Output: 6

32. 32 Comprehensions Used to generate one list from another. Can be used as shorthand for a map or foreach.

33. 33 Comprehension Example Code: raise_list(List) -> [math:pow(X,Y) || {X,Y} <- List]. Input: raise_list([{1,2},{3,4},{4,5}]) Output: [1.0,81.0,1024.0]

34. 34 Threads Erlang is incredibly efficient at creating threads. (An order of magnitude over Java) Erlang’s threading system is designed to prevent side effects –Race Condition –Deadlock The threading system is an abstraction on a message system and individual message queues.

35. 35 Performance Code: mark(0) -> {done}; mark(X) -> spawn(threadmark,thread,[]), mark(X - 1). thread() -> {ok}. This starts threads that do basically nothing, as a benchmark on thread creation. All threaded methods should be tail recursive. Non tail-recursive methods will quickly cause resource issues.

36. 36 Side Effects Race Condition –Locking Deadlock Bottom line: Erlang avoids side effects by not including the constructs that are prone to causing them.

37. 37 You’ve Got Mail Threads can receive messages. Each spawned thread has its own “mailbox”. Retrieving a message from the mailbox is triggered by the “receive” keyword. The mailbox is an abstraction over a messaging queue. The order that messages are received dictates their processing order (no way to skip, reorder, or discard messages).

38. 38 Thread Example Code: sumthread(Sum) -> io:fwrite("Waiting ~b~n",[Sum]), receive done -> io:fwrite("Was sent 'done': ~b~n",[Sum]); N -> sumthread(Sum + N) end.

39. 39 Thread Example (Cont.) Starting a thread: –Pid = spawn(module,sumthread,[0]). Sending a message: –Pid ! 12. Caveats: –Sending message to a thread that does not exist will not produce an error. –Sending a message that a thread does not have a condition for will cause the thread to end.

40. 40 IO Erlang allows developers to interact outside of the application by using IO Devices. The same methods are used for console and file IO, differing only on the device used. First, a device pointer must be obtained. – {ok, Device} = file:open(“file.txt”,[read]), Now IO can be performed on the device.

41. 41 IO Example Code: readFile(File) -> {ok, Device} = file:open(File,[read]), read(Device). read(Device) -> case io:get_line(Device, "") of eof -> file:close(Device); Line -> io:fwrite("~s",[Line]), read(Device) end.

42. 42 Easy Tokenizer Code: readFile(File) -> {ok, Device} = file:open(File,[read]), read(Device). read(Device) -> case io:get_line(Device, "") of eof -> file:close(Device); Line -> Tokens = string:tokens(Line," "), lists:map( fun (T) -> io:fwrite("~s~n",[T]) end, Tokens ), read(Device) end.

43. 43 Network IO Slightly Different than FileIO Blocking on listening Accept/Process message in another thread Binary data format.

44. 44 Network Repeater Code: % Call echo:listen(Port) to start the service. listen(Port) -> {ok, LSocket} = gen_tcp:listen(Port, ?TCP_OPTIONS), accept(LSocket). % Wait for incoming connections and spawn the echo loop when we get one. accept(LSocket) -> {ok, Socket} = gen_tcp:accept(LSocket), spawn(fun() -> loop(Socket) end), accept(LSocket). % Echo back whatever data we receive on Socket. loop(Socket) -> case gen_tcp:recv(Socket, 0) of {ok, >} -> gen_tcp:send(Socket, "done"), get_tcp:close(Socket); {ok, Data} -> gen_tcp:send(Socket, Data), loop(Socket); {error, closed} -> ok end.

45. 45 Error Handling Try / Catch Accepted form of telling a developer that something went wrong. Do not rely on status codes in failure situations. Ex: –Java: “abcd”.indexOf(“f”) returns -1 –Erlang: should throw an exception

46. 46 Homework Write a module with a function that determines the sum of all the prime numbers less than a starting number. Write a module that solves the problem: what is the sum of the digits of the number 2^(1000)? Write a module that reverses the contents of a file and writes it to the console. (Optional Bonus: to another file)

47. 47 Questions?