Erlang session1

Three sessions about Erlang Session 1 Mohamed Samy February 2010

The need for parallel processing  It has become prohibitive to raise the clock speeds of current CPUs any further.  The next trend is to increase the number of CPU cores.  In order to benefit from CPU upgrades, programs need to make the maximum possible use of parallelism.  “The free lunch is over“ (http://www.gotw.ca/publications/concurrency- ddj.htm)

Also, distributed processing :)  Not only is the program running on multiple CPUs, but on multiple computers over the network.  No shared memory, communication by message passing.  Googles index, for example.

Current languages  Both parallel and distributed applications have been written in traditional languages like C++, Java or C#, but...  The problem is these languages depend too much on shared state.  And they inherently make you think sequentially.  Your programming language influences how you think about programs, not just how you write code.

Current languages  The problem of shared mutable state (e.g global variables, object references...)  Loop iterations run in sequence  Assignment is very sequential Function calls have problems  e.g: If function a( ) calls b( ), it keeps waiting for b( ) to return before resuming operation.  ‫فيه طرق للبرمجة غير كده أصل ؟‬

Erlang  Created in 1986 by Joe Armstrong at Ericsson.  Functional, dynamically typed, uses the actor model.  Design goals:  Concurrency, distribution, robustness, "soft" real time, hot code upgrades...and other goals.  Used in Ericsson devices, Facebook chat, Amazon SimpleDB, now quickly growing.  Now open source

Contents  1st session: Sequential Erlang.  2nd session: Concurrency and Actors.  3rd session: A simple web application

Objectives of these sessions  Introducing new concepts (Actor model, functional programming, immutable data).  The importance of parallel programming.  Stressing that there is much more to know about programming outside of traditional languages like C++, Java or C#.  Not necessarily from Erlang only.

Erlang resources  Download from http://www.erlang.org/download.html  Lots and lots of documentation are included!  Located in c:Program fileserl5.7.4docindex.html  Especially look at "Getting started with Erlang" and "Erlang reference manual"  Nice, searchable version of the documentation at www.erldocs.com

The Environment  Commands always end in a dot: .  help( ) to show all options  cd(path) to change working directory Paths use a forward slash: "c:/examples/test1"  c(file) to load & compile a file. e.g: c("example1"). % .erl extension is optional  f( ) to forget all variable bindings  Simple autocomplete with the <TAB> key.  Lots and lots of documentation are included!  c:Program fileserl5.7.4docindex.html  Nice, searchable version of the documentation at www.erldocs.com

Data types  Numbers: 12, -100, 15.55, $a  Atoms: x, ayman, faculty_of_law  Simple atoms cannot begin with a capital letter or have spaces, but single quotes can be used to bypass those rules: Samy, calculus book  Tuples: { samy, 1979, cairo }, {13, 28}  Lists: [ ], [1, 2, 3], [a, [b, c], {1, 2} ]  Strings: "Hello world"  This actually is the list [$H, $e, $l, $l, $o, $ ,$w, $o, $r, $l, $d ]  All these data types are immutable

Data types and variables  More information about these data types in the Erlang reference.  Other important data types exists (e.g PID).  A piece of data of any data type is called a term.  Variables always start with a capital letter or underscore: X, Y, _Name, _name  Variables can be bound to a value or still unbound  Erlang has single assignment: a variable cannot be bound more than once.

Pattern matching  A pattern is a term which may contain one or more variables.  A term with no variables at all can be considered a pattern.  Matching syntax is:  Pattern = Term  Each respective component of the term is compared, matching attempts to find the most general variable binding to make the match succeed.  Quiz: what is the result of the following matches?

Pattern matching  5=5.  {Name, Age, _} = {“Osama”, 28, cairo }.  [1,2] = {1, 2}.  "hello" = "hello".  "ABC" = [65, 66, 67].  $a = a.

More pattern matching quizzes  X=5.  X=5. Y=X+1. Y=3+3.  X=Y. Y=1.  { X, Y } = { 5, 6 }  { Y, Y } = { 5, 6 } (When testing these on Erlang; remember to use f( ) to reset variable bindings.)

Pattern matching on lists  [ H | T ] = [1, 2, 3].  [ H | T ] = [ ].  [ H | T ] = [1, 2].  [ H1, H2 | T ] = [1, 2, 3].  [ H1, H2 | T ] = [1, 2].  [A, B] ++ [ H| T] = "faculty".  "Mustafa" ++ X = "Mustafa Kamel".

Some simple I/Oio:format(fmt_str, [arg0,arg1... ]) Examples:  io:format("The results are ~p and ~p", [15, 16]).  io:format("Hello world ~n", [ ]).  io:format("Hello world ~n"). Codes:  ~~ : The ~ mark (needs no argument)  ~f : Format argument as floating point  ~c : Format argument as a character.  ~w : Format argument in standard syntax (i.e like terms in the language)  ~p : Standard syntax for printing (e.g turns lists of printable characters into strings)  ~n : newline character (doesnt need an argument). Much more detail in the documentation. Erlang has very rich formatting features.

More simple I/Oio:get_line(Prompt) -> Data | eof | {error,Reason}  Gets a line from standard input as a string (includes the newline character).io:read(Prompt) → {ok, Term} | eof | {error, ErrorInfo}  Reads a term from standard input (user must include ending period).io:fread(Prompt, Format) -> {ok, Terms} | eof | {error, What}  Reads characters and returns a list of terms, parsed according to the specified format specification (different from that of io:format). Examples:  io:read("Enter a point>"). Enter a point>{13, 14}. {ok, {13, 14}}  io:get_line("Enter your name>"). Enter your name>Captain Majid “Captain Majidn" io:format( ), io:read( ) and all the other given i/o functions have additional parameters that can make them read from I/O devices like files...etc

Modules -module(addition). -export([add/2, add/3]). add(X, Y) -> X+Y. add(X, Y, Z) -> X+Y+Z. unused_func( ) -> io:format("unused!"). % End of module definition.  Erlang code is divided into modules.  A module has attributes at the top (the begin with the dash character -) and is followed by function declarations. Both attributes and declaration end with a dot .  Exported function names must include its artiy (number of parameters).  Two functions with the same name but different arities are different functions!

Functions  add(A, B) → A+B. − Return value is the functions expression  add(A, B) → io:format(“now adding ~p and ~p~n", [A, B]), A+B. − In case of multiple expressions, return value is the last one

Functions  absolute(A) when A>0 → 1 ; absolute(A) when A=0 → 0 ; absolute(A) → -1 .  The expressions after when are called guard sequences. More about them in the Erlang language reference → Expressions → Guard sequences

Pattern matching in functions  distance({X1 Y1}, {X2, Y2}) → Dx= X1-X2, Dy= Y1-Y2, math:sqrt(Dx*Dx + Dy*Dy).  Tuples can be confusing (e.g is a tuple of two numbers a point, vector or age & salary?).  We can use an atom in the beginning (the tag) to distinguish between kinds of data...

Pattern matching in functions  distance({point, X1, Y1}, {point, X2, Y2}) → Dx= X1-X2, Dy= Y1-Y2, math:sqrt(Dx*Dx + Dy*Dy).  example:distance({point, 0, 0}, {point, 100, 100}).

Recursion factorial(0) ->1 ; factorial(N) -> N*factorial(N-1).  Function calls, activation records, and why this works.  We need this; we dont have loops !  The space requirements of naïve recursion.

Recursion  Tail calls vs. non tail calls.  Tails calls are not only about recursion, but any function call.  Tail call elimination. Now recursion can be as good as loops.  In fact, its can be compiled to jump instructions.  Sometimes it can be much better than loops!  Example: state machines

Next...  Processes!  Actors!  Ping pong! − Thursday, 11 February 2010.

Erlang session1

by mohamedsamyali

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