The Erlang Programming Language Dennis Byrne - ThoughtWorks [email_address] http://notdennisbyrne.blogspot.com/ © ThoughtWorks 2008

Introduction Not a “shiny new object (or function)” Open sourced by Ericsson Functional Programming Concurrency Reliability ThoughtWorks ! Erlang © ThoughtWorks 2008 Erlang is a functional programming language, with native constructs for concurrency and reliability.

Not a “shiny new object (or function)”

Open sourced by Ericsson

Functional Programming

Concurrency

Reliability

ThoughtWorks ! Erlang

A Totally “Unbiased” Comparison In Erlang: List = [1,2,3,4,5]. Doubled = lists:map( fun(I) -> I * 2 end, List). In Java: List<Integer> list = new LinkedList<Integer>() {{ add(1); add(2); add(3); add(4); add(5); }}; List<Integer> doubled = new ArrayList<Integer>(); for(Integer integer : list) { doubled.add(integer.intValue() * 2); } © ThoughtWorks 2008

In Erlang:

List = [1,2,3,4,5].

Doubled = lists:map( fun(I) -> I * 2 end, List).

In Java:

List<Integer> list = new LinkedList<Integer>() {{

add(1); add(2); add(3); add(4); add(5);

}};

List<Integer> doubled = new ArrayList<Integer>();

for(Integer integer : list) {

doubled.add(integer.intValue() * 2);

}

Single Assignment & Unification I = 4. % assignment I = 4. % unification I = I. % unification 4 = I. % unification I = 5. % throws error :( © ThoughtWorks 2008

I = 4. % assignment

I = 4. % unification

I = I. % unification

4 = I. % unification

I = 5. % throws error :(

Pattern Matching 1> MyList = [ “ A ” ]. % declaring a list 2> [Var] = MyList. % implicit declaration 3> io:format(Var). % prints “ A ” 1> MyList = [ “ A ” , “ B ” ]. % declaring a list 2> [_, Var] = MyList. % _ is ignored 3> io:format(Var). % prints B © ThoughtWorks 2008

1> MyList = [ “ A ” ]. % declaring a list

2> [Var] = MyList. % implicit declaration

3> io:format(Var). % prints “ A ”

1> MyList = [ “ A ” , “ B ” ]. % declaring a list

2> [_, Var] = MyList. % _ is ignored

3> io:format(Var). % prints B

Functions as First Class Citizens 1> Msg = &quot;I am scoped&quot;. 2> F = fun()-> io:format(Msg) end. 3> F(). % prints “ I am scoped ” © ThoughtWorks 2008

1> Msg = &quot;I am scoped&quot;.

2> F = fun()->

io:format(Msg)

end.

3> F(). % prints “ I am scoped ”

Tail Recursion Tail recursion is to ‘recursion’ as optimistic locking is to ‘locking’. loop() -> io:format( “ in constant space ” ), loop() . Automatic byte code manipulation Last call optimization © ThoughtWorks 2008

Tail recursion is to ‘recursion’ as optimistic locking is to ‘locking’.

loop() ->

io:format( “ in constant space ” ),

loop() .

Automatic byte code manipulation

Last call optimization

Pop Quiz: Evaluation Strategies f( a(), b() ) Order of evaluation Lazy or strict? Left, right, both, neither? © ThoughtWorks 2008

f( a(), b() )

Order of evaluation

Lazy or strict?

Left, right, both, neither?

State Erlang avoids complex solutions in favor of simple ways to avoid the problem State is immutable w/ single assignment State is private, passed by value Changing Perspectives Contention vs. Scale Side Effects vs. Scale © ThoughtWorks 2008

Erlang avoids complex solutions in favor of simple ways to avoid the problem

State is immutable w/ single assignment

State is private, passed by value

Changing Perspectives

Contention vs. Scale

Side Effects vs. Scale

Erlang Process An Erlang process combines the best of An operating system process An operating system thread A “green thread” Controlled in user space by the ERTS Costs less than 300 bytes Private heap Private stack All processes are created equal Incremental garbage collection © ThoughtWorks 2008

An Erlang process combines the best of

An operating system process

An operating system thread

A “green thread”

Controlled in user space by the ERTS

Costs less than 300 bytes

Private heap

Private stack

All processes are created equal

Incremental garbage collection

The Actor Model Asynchronous Message passing Messages passed by value, not reference One to one relationship An Actor A mailbox A Process A Process ID, or pid © ThoughtWorks 2008

Asynchronous Message passing

Messages passed by value, not reference

One to one relationship

An Actor

A mailbox

A Process

A Process ID, or pid

Concurrency Primitives: spawn A built in function used to create a process. Pid = spawn(Node, Module, Function, Args) © ThoughtWorks 2008

A built in function used to create a process.

Pid = spawn(Node, Module, Function, Args)

Another “Unbiased” Comparison In Erlang: Pid = spawn(fun() -> io:format(“hello world”) end). In Java: Thread thread = new Thread() { public void run() { System.out.println(&quot;Hello World&quot;); } }.start(); © ThoughtWorks 2008

In Erlang:

Pid = spawn(fun() -> io:format(“hello world”) end).

In Java:

Thread thread = new Thread() {

public void run() {

System.out.println(&quot;Hello World&quot;);

}

}.start();

Concurrency Primitives: send ! © ThoughtWorks 2008

!

Concurrency Primitives: send The send operator passes a message to a process. It is “fire and forget”. Pid ! Msg © ThoughtWorks 2008

The send operator passes a message to a process. It is “fire and forget”.

Pid ! Msg

Concurrency Primitives: receive receive Pattern1 [ when Guard1 ] -> dosomething(). Pattern2 [ when Guard2 ] -> dosomethingelse(). end © ThoughtWorks 2008

receive

Pattern1 [ when Guard1 ] ->

dosomething().

Pattern2 [ when Guard2 ] ->

dosomethingelse().

end

Reliability catch try Monitor monitor_node(Node, Bool) Unidirectional Link Pid = spawn_link(Node, Module, Fun, Args) Bi-directional © ThoughtWorks 2008

catch

try

Monitor

monitor_node(Node, Bool)

Unidirectional

Link

Pid = spawn_link(Node, Module, Fun, Args)

Bi-directional

The Opposite of Ruby © ThoughtWorks 2008 Erlang Ruby Orientation Functional Object Syntax Ugly Beautiful Concurrent Good Bad Track Record Back end Front end Execution Model Compiled Interpreted

Erlang Servers and Frameworks Mnesia – distributed database YAWS – “yet another web server” ErlyWeb – MVC web framework EUnit Jungerl – third party libraries Perferl ( http://code.google.com/p/perferl ) © ThoughtWorks 2008

Mnesia – distributed database

YAWS – “yet another web server”

ErlyWeb – MVC web framework

EUnit

Jungerl – third party libraries

Perferl ( http://code.google.com/p/perferl )

The Erlang Programming Language Dennis Byrne - ThoughtWorks [email_address] http://notdennisbyrne.blogspot.com/ © ThoughtWorks 2008

Dennis Byrne - ThoughtWorks

[email_address]

http://notdennisbyrne.blogspot.com/