Introduction to Erlang

Systèmes Distribués Eddy Caron Introduction to Erlang

Introduction to Erlang

Eddy Caron

2013M1. ENS-Lyon

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Piece of History

1982 – 1985 Experiments with programming of telecom using > 20 different languages. Conclusion: The language must be a very high level symbolic language in order to achive productivity gains ! (Leaves us with: Lisp , Prolog , Parlog ...) 1985 – 86 Experiments with Lisp, Prolog, Parlog etc. Conclusion: The language must contain primitives for concurrency and error recovery, and the execution model must not have back-tracking. (Rules out Lisp and Prolog). We must therefore develop our own language with the desirable features of Lisp, Prolog and Parlog, but with concurrency and error recovery built into the language.1987 The first experiments with Erlang. 1993 Distribution is added to Erlang, which makes it possible to run a homgeneous Erlang system on a heterogeneous hardware. Decision to sell implementations Erlang externally. Separate organization in Ericsson started to maintain and support Erlang implementations and Erlang Tools.

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The Erlang Shell

The Erlang Shell

You can use Ctrl+c to break

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[ecaron@aspen bin]$ ./erlErlang R15B02 (erts-5.9.2) [source] [smp:8:8] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.9.2 (abort with ^G)1> 2+5.72> (42+6)*33/4.396.03> halt().[ecaron@aspen bin]$

BREAK: (a)bort (c)ontinue (p)roc info (i)nfo (l)oaded (v)ersion (k)ill (D)b-tables (d)istribution


Hello world

‘%’ starts a comment‘.’ ends a declarationEvery function must be in a module

one module per source filesource file name is module name + “.erl”

‘:’ used for calling functions in other modules

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Module and Functions

A programming language isn't much use if you can just run code from the shell.

use it

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[ecaron@aspen Erlang]$ cat mult2.erl -module(mult2).-export([double/1]).

double(X) -> 2 * X.

[ecaron@aspen Erlang]$ erlErlang R15B02 (erts-5.9.2) [source] [smp:8:8] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.9.2 (abort with ^G)1> c(mult2).{ok,mult2}2> mult2:double(6). 12

Compilation is ok !


Numbers

Regular numbers

#-notation for base-N integers

$-notation for character codes (ISO-8859-1)

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123-3456712.345 -27.45e-05

1> 16#ff.255

2> $A.65


Atoms

<!!>Must start with lower case character or be quoted

Similar to hashed stringsuse only one word of data constant-time equality test

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fridayunquoted_atoms_cannot_contain_blanks’A quoted atom with several blanks’’hello \n my friend’


Tuples

Terms seprated by ‘,’ and enclosed in {}

A fixed number of items (similar to structure or record in conventional programming languages) A tuple whose first element is an atom is called a tagged tuple

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{123, bcd}{123, def, abc}{person, 'Jax', ’Teller'}{abc, {def, 123}, jkl}{}


Other Data Types

FunctionsBinariesProcess identifiers References: A reference is a term which is unique in an Erlang runtime system, created by calling make_ref/0.Erlang values in general are called « terms »All terms are ordered and can be compared with ‘<’, ‘>’, ‘==’, ‘=:=’ , etc. 9

Module:fun(Arg1,Arg2,… Argn)

Bin = <<Bin0,...>>

1> spawn(m, f, []).<0.51.0>


Recursive Functions

Variables start with upper-case characters‘;’ separates function clauses‘,’ separates instructionsVariables are local to the function clause Pattern matching and guards to select clauses

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Compile and run

recurs.erl

Recursive Functions

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-module(recurs).-export([fac/1]). fac(0) -> 1; fac(N) -> N * fac(N-1).


Eshell V5.9.2 (abort with ^G)1> c(recurs).{ok,recurs}2> recurs:fac(6).720


dblfunc.erl

Recursive Functions

Compile and run

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-module(dblfunc).-export([fac/1, mult/2]).

fac(1) -> 1;fac(N) -> N * fac(N - 1).

mult(X, Y) -> X * Y.


Eshell V5.9.2 (abort with ^G)1> c(dblfunc).{ok,dblfunc}2> dblfunc:fac(4).243> dblfunc:mult(dblfunc:fac(4),2).48


The arity is part of the function nameNon-exported functions are local to the module

mylists.erl

Tail Recursion

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-module(mylists).-export([reverse/1]).reverse(L) -> reverse(L, []).reverse([H|T], L) -> reverse(T, [H|L]);reverse([], L) -> L.

> mylists:reverse([3,2,1]).[1,2,3]


Tail Recursion

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> mylists:reverse([1,2,3]).[3,2,1]

reverse([1,2,3])->reverse([1,2,3],[]).->reverse([2,3],[1])->reverse([3],[2,1])->reverse([],[3,2,1])->[3,2,1]

mylists.erl-module(mylists).-export([reverse/1]).reverse(L) -> reverse(L, []).reverse([H|T], L) -> reverse(T, [H|L]);reverse([], L) -> L.


Recursion over Lists

Pattern-matching selects components of the data <!!!> ‘_’ is a “don’t care” pattern (not a variable)‘[]’ is the empty list ‘[X,Y,Z]’ is a list with exactly three elements ‘[X,Y,Z|Tail]’ has three or more elements

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List Recursion with Accumulator

The same syntax is used to construct listsStrings are simply lists of character codesAvoid adding data to the end of the list

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Built-in Functions

Implemented in C All the type tests and conversions are BIFs Most BIFs (not all) are in the module “erlang” Many common BIFs are auto-imported (recognized without writing “erlang:...”) Operators (‘+’,’-’,’*’,’/’,...) are also really BIFsDescribed in the BIFS manual

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Built-in Functions

Some examples

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1> date().{2012,10,23}2> time().{1,14,35}3> length([1,2,3,4,5]).54> size({a,b,c}).35> atom_to_list(an_atom)."an_atom"6> list_to_tuple([1,2,3,4]).{1,2,3,4}7> integer_to_list(3412)."3412"8> tuple_to_list({}).[]


Application LibrariesKernel

a. erlangb. codec. filed. inete. os

Stdliba. listsb. dictc. setsd. ...

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Standard Libraries


Expressions

Boolean and/or/xor are strict (always evaluate both arguments) Use andalso/orelse for short circuit evaluation‘==’ for equality, not ‘=’ Always use parentheses when not absolutely certain about the precedence

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Fun Expressions

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1> Fun1 = fun (X) -> X+1 end.#Fun<erl_eval.6.39074546>2> Fun1(2).33> Fun2 = fun (X) when X>=5 -> gt; (X) -> lt end.#Fun<erl_eval.6.39074546>4> Fun2(7).gt


Pattern Matching

Match failure causes run-time errorSuccessful matching binds the variables

but only if they are not already bound to a value previously bound variables can be used in a patterna new variable can also be repeated in a pattern

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-module(pm).-export([mylength/1]).mylength([]) -> 0;mylength([_|T]) -> mylength(T) + 1.


Case-switches

Any number of clausesPatterns and guards, just as in functions‘;’ separates clausesUse ‘_’ as catch-allVariables may also begin with underscore

signals “I don’t intend to use this value”

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is_valid_signal(Signal) -> case Signal of {signal, _What, _From, _To} -> true; {signal, _What, _To} -> true; _Else -> false end.


If-switches

Like a case-switch without the patterns and the ‘when’ keywordUse ‘true’ as catch-all

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factorial(N) when N == 0 -> 1;

factorial(N) when N > 0 -> N * factorial(N - 1).


Pattern Matching

When

If

Case

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Switching

factorial(0) -> 1; factorial(N) -> N * factorial(N-1).

factorial(N) when N == 0 -> 1; factorial(N) when N > 0 ->

N * factorial(N - 1).

factorial(N) -> ifN == 0 -> 1; N > 0 -> N * factorial(N - 1) end.

factorial(N) -> 1 case (N) of

0 -> 1; N when N > 0 -> N * factorial(N - 1) end.


List Processing BIFs List Processing Functions

Tuple Processing BIFS

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List and Tuple Processing

atom_to_list(A)float_to_list(F)integer_to_list(I)tuple_to_list(T)list_to_atom(L)...hd(L)tl(L)length(L)

member(X,L)append(L1,L2)reverse(L)delete_all(X,L)

tuple_to_list(T)element(N,T)setelement(N,T,Val)size(L)…


Catching Exceptions

throw: user definederror: runtime errorsexit: end processonly catch throw exceptionsnormally

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-module(catch_it).-compile(export_all). % An example of throw and catch g(X) when X >= 13 -> ok;g(X) when X < 13 -> throw({exception1, bad_number}). % Throw in g/1 % Catch in start/1 start(Input) -> case catch g(Input) of {exception1, Why} -> io:format("trouble is ~w ", [ Why ]); NormalReturnValue -> io:format("good input ~w ", [ NormalReturnValue ] ) end.

8> c(catch_it). {ok,catch_it} 9> catch_it:start(12).trouble is bad_number ok 10> catch_it:start(13).good input ok ok


Processes

Code is executed by a processA process keeps track of the program pointer, the stack, the variables values, etc. Every process has a unique process identifier: PIDProcesses are concurrentVirtual machine layer processesPreemptive multitaskingLittle overhead (e.g. 100.000 processes) Can use multiple CPUs on multiprocessor machines

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Concurrency

• Several processes may use the same program code at the same time

– each has own program counter, stack, and variables – programmer need not think about other processes

updating the variables

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Message Passing

• “!” is the send operator– Pid of the receiver is used as the address

• Messages are sent asynchronously – The sender continues immediately

• Any value can be sent as a message

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1> c(echo).{ok,echo}2> Id=echo:start(), 2> Id ! {self(),hello}.> echo: <0.38.0> Msg: hello {<0.31.0>,hello}

echo.erl-module(echo).-export([start/0,loop/0]).start() -> spawn(echo, loop, []).loop() -> receive {From, Message} -> io:format("> echo: ~w Msg: ~w ~n", [self(), Message]), From ! Message, loop()end.


Message Queues

• Each process has a message queue (mailbox)– incoming messages are placed in the queue (no size

limit) • A process receives a message when it extracts it

from the mailbox – need not take the first message in the queue

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Receive a Message

• receive-expressions are similar to case switches – patterns are used to match messages in the mailbox – messages in the queue are tested in order– only one message can be extracted each time

• Selective receive– Patterns and guards permit message

selection – receive-clauses are tried in order – If no message matches, the process

suspends and waits for a new message 32


Receive with Timeout

• A receive-expression can have an after-part– can be an integer (milliseconds) or “infinity”

• The process waits until a matching message arrives, or the timeout limit is exceeded

– soft real-time: no guarantees

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sleep(T)- process suspends for T ms.sleep(T) ->

receive after T -> true end.

suspend() - process suspends indefinitely.suspend() ->

receiveafter infinity -> trueend.


Send and Reply

• Pids are often included in messages (self()), so that the receiver can reply to the sender

– If the reply includes the Pid of the second process, it is easier for the first process to recognize the reply

• Message order– The only guaranteed message order is

when both the sender and the receiver are the same for both messages (first-in, first- out)

• Selecting Unordered Messages– Using selective receive, it is possible to choose which messages to accept,

even if they arrive in a different order

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Starting Processes

• The “spawn” function creates a new process • The new process will run the specified function

– The spawn operation always returns immediately• The return value is the Pid of the “child”

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Ping Pong

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tut16.erl-module(tut16).-export([start/0, ping/1, pong/0]).ping(0) -> pong ! finished, io:format("ping finished~n", []);ping(N) -> pong ! {ping, self()}, receive pong -> io:format("Ping received pong~n", []) end, ping(N - 1).pong() -> receive finished -> io:format("Pong finished~n", []); {ping, Ping_PID} -> io:format("Pong received ping~n", []), Ping_PID ! pong, pong() end.start() -> register(pong, spawn(tut16, pong, [])), spawn(tut16, ping, [3]).

1> c(tut16).{ok,tut16}2> tut16:start().Pong received ping<0.39.0>Ping received pongPong received pingPing received pongPong received pingPing received pongping finishedPong finished


Process Termination

• Process Termination A process terminates when:– it finishes the function call that it started with

a. There is an exception that is not caught

• All messages sent to a terminated process will be thrown away

• Same Pid will not be used before long time

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Registered Processes

• A process can be registered under a name • Any process can send a message to a registered process,

or look up the Pid • The Pid might change (if the process is restarted and re-

registered), but the name stays the same

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Pid = spawn(?MODULE, server, []),register(myserver, Pid),myserver ! Msg.


Links and Exit Signals

• Any two processes can be linked– Links are always bidirectionnal

• When a process dies, an exit signal is sent to all linked processes, which are also killed

– normal exit does not kill other processe• If a process sets its trap_exit flag, all signals will be

caught and turned into normal messages – process_flag(trap_exit, true) – Signal is turned into message {‘EXIT’, Pid, ErrorTerm}

• This way, a process can watch other processes 39


Distribution

• Running “erl” with the flag “-name xxx”– starts the Erlang network distribution system– Makes the virtual machine emulator a “node” (‘[email protected]’)

• Erlang nodes can communicate over the network (but must find each other first)

• Possible to send a Pid from one node to another (Pids are unique across nodes)

• You can send a message to any process through its Pid (even on another node)

• You can run several Erlang nodes (with different names) on the same computer

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Connecting Nodes

• Nodes are connected the first time they try to communicate

• The function “net_adm:ping(Node)” is the easiest way to set up a connection between nodes

– returns “pong” or “pang” • Send a message to a registered process using

– “{Name,Node} ! Message”

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Running Remote Processes

• Variants of the spawn function can start processes directly on another node

• The module ‘global’ contains functions for – registering and using named processes over the whole

network of connected nodes • setting global locks

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Ports:Talking to the Outside

• Talks to an external (or linked-in) C program • A port is connected to the process that opened it • The port sends data to the process in messages • A process can send data to the port

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Conclusion

• Try Erlang in practical session…• … and send me your own conclusion

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References

• Special and greatful thanks to Franck Petit et Sebastien Tixeuil• http://www.erlang.org• http://en.wikibooks.org/wiki/Erlang_Programming

• Use case pour normalien ;-)– Un Caml Light Distribué [Elkamel Merah , Allaoua Chaoui]– « … Pour l’extension concurrente de Caml Light nous proposons quelques

primitives avec une semantique tres simple en utilisant le modele du langage de programmation ERLANG. Le support de la creation dynamique de processus et de la communication asynchrone entre processus sont les deux principales extensions du langage Caml Light. "

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Introduction to Erlang