Introduction to ErlangGabriele Lana

gabriele.lana@gmail.com

Why Erlang?

Damien Katz @ Ruby Fringe 2008

Erlang (BEAM) emulator version 5.5.6 [source][smp:1] [async-threads:0] [hipe] [kernel-poll:true]

Eshell V5.5.6 (abort with ^G)1>

Erlang Shell

1> 3.3

2> 1 + 2008.2009

3> 849274893217458392017483921 * 78932489321748392107483291.67035381440116040485849541847226463097212333368664011

Syntax: Number

1> atom.atom

2> anythingStartingWithLowercaseLetter. anythingStartingWithLowercaseLetter

3> 'anything enclosed in single quotes'.'anything enclosed in single quotes'

Syntax: Atom

1> true.true

2> false.false

3> is_atom(true).true

4> is_boolean(true).true

Syntax: Boolean

1> "foo"."foo"

2> "anything enclosed in double quotes"."anything enclosed in double quotes"

Syntax: String

1> [].[]

2> [ 1 ].[1]

3> [ first ].[first]

4> [ "first" ].["first"]

5> [ 1, first, "first" ].[1,first,"first"]

Syntax: List

1> [ 72, 101, 108, 108, 111 ]."Hello"

2> $H.72

3> [ $H, $e, $l, $l, $o ]."Hello"

Syntax: Strings are Lists

1> { 1, 2, 3 }.{1,2,3}

2> { name, "Gabriele" }.{name,"Gabriele"}

3> { coder, { name, "Gabriele" }, { language, "Erlang" } }.{coder,{name,"Gabriele"},{language,"Erlang"}}

Syntax: Tuple

1> 1 = 1.1

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

3> catch 1 = 2.{'EXIT',{{badmatch,2},[{erl_eval,expr,3}]}}

Pattern Matching

1> X.* 1: variable 'X' is unbound

2> X = 5.5

3> X.5

4> X = 6.** exception error: no match of right hand side value 6

Pattern Matching: Binding Variables

1> Coder = { coder, { name, "Gabriele" } }.{coder,{name,"Gabriele"}}

2> { person, { name, PersonName } } = Coder.** exception error: no match of right hand side value {coder,{name,"Gabriele"}}

3> { coder, { name, "Gabriele" } } = Coder.{coder,{name,"Gabriele"}}

4> { coder, { name, CoderName } } = Coder. {coder,{name,"Gabriele"}}

Pattern Matching: Destructuring

7> AnotherCoderName = "Matteo"."Matteo"

8> { coder, { name, AnotherCoderName } } = Coder.** exception error: no match of right hand side value {coder,{name,"Gabriele"}}

Pattern Matching: Destructuring

1> [ First, Last ] = [ first, last ].[first,last]

2> First.first

3> Last.last

Pattern Matching: List

4> [ Head | Tail ] = [ 1, 2, 3, 4, 5 ]. [1,2,3,4,5]

5> Head.1

6> Tail.[2,3,4,5]

Pattern Matching: List

1> [ First, Second | Others ] = [ 1, 2, 3, 4, 5 ].[1,2,3,4,5]

2> First.1

3> Second.2

4> Others.[3,4,5]

Pattern Matching: List

1> { X, Y, X } = { { abc, 12 }, 42, { abc, 12 } }.{{abc,12},42,{abc,12}}

2> X.{abc,12}

3> Y.42

Pattern Matching: In Deep

1> { X, Y, X } = { { abc, 12 }, 42, true }.** exception error: no match of right hand side value {{abc,12},42,true}

2> { X, Y, _ } = { { abc, 12 }, 42, true }.{{abc,12},42,true}

Pattern Matching: In Deep

-module(examples).-export([ hello/0 ]).

hello() -> "Hello World!".

1> c(examples).{ok,examples}

2> examples:hello()."Hello World!"

Function

Syntax: Punctuation

• Periods (.) end everything except when

• Semicolons (;) end clauses

• and Commas (,) separate expressions

-export([ double/1 ]).

double(0) -> 0;double(Number) -> Number * 2.

1> examples:double(0).0

2> examples:double(4).8

Function: Pattern Mathing & Clause

double(0) -> 0;double(Number) when is_integer(Number) -> Number * 2.

1> examples:double(4).8

2> examples:double(foo).** exception error: no function clause matching examples:double(foo)

Function: Guard

double(0) -> 0;double(Number) when is_integer(Number) -> Number * 2;double(AnythingElse) -> io:format("I don't know how to double ~p~n", [ AnythingElse ]), error.

1> examples:double(foo).I don't know how to double fooerror

Function: Guard

multiplyBy(Multiplier) -> fun(Number) -> Number * Multiplier end.

1> Double = examples:multiplyBy(2).#Fun<examples.0.46516809>2> Double(4).83> MultiplyBy4 = examples:multiplyBy(4).#Fun<examples.0.46516809>4> MultiplyBy4(4).16

Function: Anonymous & Closure

printList([]) -> ok;printList([ Item | List ]) -> io:format("~p~n", [ Item ]), printList(List).

1> examples:printList([ 1, 2, 3, 4 ]).1234ok

Function: Tail Recursion

foreach(_Function, []) -> ok;foreach(Function, [ Item | List ]) -> Function(Item), foreach(Function, List).

1> examples:foreach(fun(Item) -> io:format("~p~n", [ Item ])end, [ 1, 2, 3, 4 ]).1234

Function: High Order Function

foreach(Function, List) -> lists:foreach(Function, List).

1> examples:foreach(fun(Item) -> io:format("~p~n", [ Item ])end, [ 1, 2, 3, 4 ]).1234ok

Function: High Order Function

3> lists:foreach(fun(Item) -> io:format("~p~n", [ Item ])end, [ 1, 2, 3, 4 ]).1234ok

Function: High Order Function

names(Coders) -> lists:map(fun({ coder, { name, CoderName } }) -> CoderName end, Coders).

1> examples:names([ 1> { coder, { name, "Gabriele" } },1> { coder, { name, "Matteo" } } 1> ]).["Gabriele","Matteo"]

List Functions: Map

triplets(UpTo) when is_number(UpTo), UpTo >= 2 -> [ { A, B, C } || A <- lists:seq(2, UpTo), B <- lists:seq(2, UpTo), C <- lists:seq(2, UpTo), A * A + B * B =:= C * C ].

1> examples:triplets(10).[{3,4,5},{4,3,5},{6,8,10},{8,6,10}]

Syntax: List Comprehension

sum(Numbers) -> lists:foldl(fun(Number, Sum) -> Sum + Number end, 0, Numbers).

1> examples:sum([ 1, 2, 3, 4, 5 ]).15

List Functions: Fold/Reduce

sum(Numbers) -> lists:foldl(fun (Number, Sum) when is_integer(Number) -> Sum + Number; (_Number, Sum) -> Sum end, 0, Numbers).

1> examples:sum([ 1, 2, 3, foo, 4, bar, 5 ]).15

List Functions: Fold/Reduce

fizzbuzz(UpTo) -> lists:map(fun(Counter) -> if (Counter rem 15) =:= 0 -> fizzbuzz; (Counter rem 3) =:= 0 -> fizz; (Counter rem 5) =:= 0 -> buzz; true -> Counter end end, lists:seq(1, UpTo)).

1> examples:fizzbuzz(16).[1,2,fizz,4,buzz,fizz,7,8,fizz,buzz,11,fizz,13,14,fizzbuzz,16]

Example: FizzBuzz

Process: Concurrency

• Any function (named or anonymous) can become a process

• A process is simply a function executing in parallel

• A process shares nothing with other processes• A process is extremely lightweight: a modern

systems can accommodate literally millions of Erlang processes

1> processes:profileSpawnFor(1000). Spawned 1000 processes in 10 (4) millisecondsok

2> processes:profileSpawnFor(10000). Spawned 10000 processes in 40 (96) millisecondsok

3> processes:profileSpawnFor(100000).Spawned 100000 processes in 510 (884) millisecondsok

Process: Concurrency

Process: Concurrency

Process: Concurrency

• The “!” operator sends messages• Asynchronous• Order guaranted

• The “receive” statement matches messages• One call to receive, one message removed

from the process message queue• Uses pattern matching to select messages

1> Pid = spawn(fun() ->1> receive1> die -> io:format("Bye~n")1> end1> end).<0.33.0>

Process: Concurrency

2> is_process_alive(Pid).true

3> Pid ! die.Byedie

4> is_process_alive(Pid).false

Process: Concurrency

profileSpawnFor(Number) -> Processes = for(1, Number, fun() -> spawn(fun() -> wait() end) end), lists:foreach(fun(Pid) -> Pid ! die end, Processes).

wait() -> receive die -> void end.

Process: Concurrency

Process: Fault Tollerance

• The “link” function can link processes• When a process die the linked processes

receive a message { ‘EXIT’, Died, Reason } • A monitor process could respawn the died

ones or cleanup the system

Process: Distributed

• The spawn(Node, Function) can spawn a process in any other known nodes

• No other changes are required!!!

Hard Gained Insights

• Don’t miss tail-call recursion• Asynchronous is good• Small functions are good• When in doubt create a new process• Think early what to do when a process die

Yaws(Yet Another Web Server)

eJabbered(jabber/XMPP)

Disco(Map/Reduce with Erlang + Python)