Erlang

http://erlang.org/

Noah Dietrich

Chris Ruffalo

2

Introduction to Erlang

• Designed by Ericson for phone switches.

• Released Open Source in 1998.

• Interpreted VM code.

3

Strengths & Uses

• Distributed, fault-tolerant, soft-real-time.

• Hot-swap code

• Lightweight threading, SMP.

4

Using Erlang

• Open Source – Download from http://erlang.org

• Read-Eval loop

• Run Erlang from your source directory:

5

Variables: Numbers

• Terms

• Integer

• Float

6

Examples: Numbers

>1 + 1.

2

>1000000000000000000000 + 3.

1000000000000000000003

> .1 +2.

2.1

7

Variables: Continued

• Atoms

• Tuples

• Lists

• Bit String

8

ExamplesExample 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

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

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

Advanced Types

• PID (process ID)

• Port

• Reference

• Fun (anonymous function)

12

Not Types

• Boolean

• Char

• String

• Record

13

ExamplesLists:

"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

Control Statements

• If

• Case

• Guards

• No For-loop

15

Guards

• Used in conditional statements.

• Guard Expression:

• Guard

• Guard Sequence

16

Guard ExamplesGuard 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

If Expression

if

GuardSeq1 ->

Body1;

...;

GuardSeqN ->

BodyN;

true ->

% works as an else branch

end.

18

Case Expression

case Expr of

Pattern1 [when GuardSeq1] ->

Body1;

...;

PatternN [when GuardSeqN] ->

BodyN

_Else ->

BodyElse;

end.

19

For Loop

• Erlang does not support looping

• Use recursive function calls instead.

20

Memory Management

• Garbage Collection

• Threads and Memory Overhead

• Activation Records

21

Modules

• Contain attributes and functions

• Expose interface

• Compiled into .beam files (object code)

• Compiled & loaded from interpreter with – > c(Module)

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

Functions

• Pattern match implementation

• First-class functions

• Tail Recursion

• Use Guard Sequences to identify correct clause

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

Functional Object (Anonymous Functions)

1> Fun1 = fun (X) -> X+1 end.

#Fun<erl_eval.6.39074546>

2> Fun1(2).

3

26

List Operations

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

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

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

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

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

Folding Example

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

32

Comprehensions

• Used to generate one list from another.

• Can be used as shorthand for a map or foreach.

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

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

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

Side Effects

• Race Condition– Locking

• Deadlock

• Bottom line: Erlang avoids side effects by not including the constructs that are prone to causing them.

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

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

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

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

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

Easy TokenizerCode: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

Network IO

• Slightly Different than FileIO

• Blocking on listening

• Accept/Process message in another thread

• Binary data format.

44

Network RepeaterCode:% 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, <<"quit\n">>} -> gen_tcp:send(Socket, "done"), get_tcp:close(Socket); {ok, Data} -> gen_tcp:send(Socket, Data), loop(Socket); {error, closed} -> ok end.

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

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

Questions?