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.

Google's 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/O
io: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 (doesn't need an argument).

Much more detail in the documentation. Erlang has very rich formatting
features.

More simple I/O
io: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 function's 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 don't 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, it's 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

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