Introduction to Erlang Part 1

What Is Erlang?What Is Erlang?
Erlang is a programming language for building highly parallel,
distributed, fault-tolerant systems.
To start the Erlang interpreter, type erl at the shell prompt. And do not
forget: there is a dot at the end of an Erlang statement, not a
semicolon.

Facts about ErlangFacts about Erlang
● “Shared-nothing” model: no shared variables, pure message passing
● Erlang processes run on nodes, nodes run on physical computers
● Pairs of processes can be linked to support termination notification
● Hot-swappable modules (dynamic on-the-fly code replacement)
● Mnesia: built-in distributed fault-tolerant database
● OTP (Open Telecom Platform), a set of libraries and procedures that
includes:
● A complete Web server
● A complete FTP server
● A CORBA ORB

Apach vs YawsApach vs Yaws
● Yaws is an Erlang-
based Web server
● The graph shows
thoughput (KB/sec)
vs. load.
● Apache (blue and
green) dies when
subject to a load of
ca. 4000 parallel
sessions.

Erlang execution modelErlang execution model

Sequential ErlangSequential Erlang

Programming Erlang: the Sequential Part 6
ArithmeticsArithmetics
● Erlang integer numbers are arbitrary-sized (there is no overflow)
● 16#19AC is a base 16 number, 32#SUGAR is a base 32 number, etc.
● / always produces a floating point value; rem and div perform integer
division. 5/2 is 2.5
● bnot, band, bor, bxor, bsl, bsl perform bitwise operations

VariablesVariables
● Variables in Erlang are single-assignment (immutable), like final in
Java. Single assignment does not cause side effects and makes
parallelism easy. Variable names begin with an uppercase letter.
● = is not assignment but matching: it matches the LHS against the RHS
and binds variables to the matching values. No weird math like
X=X+1.
● Function f() “forgets” all existing bindings.

ExamplesExamples
1> Oranges = 7.
7
2> Apples = 12.
12
3> Apples1 = Apples / 5 + Oranges.
9.4
4> Apples = Apples.
12
5> Oranges = Apples.
** exception error: no match of right hand side value
12

AtomsAtoms
● An atom is like a C/Java enum. It begins with a lowercase letter, can
consist of alphanumeric symbols, underscores _, and @. It can also be
any sequence of symbols in single quotation marks.
● The value of an atom is the atom itself.

ExamplesExamples
1> Apple.
* 1: variable 'Apple' is unbound
2> apple.
apple
3> 'Apple'.
'Apple'
4> apples@oranges.
apples@oranges.

TuplesTuples
● A tuple is a collection of values enclosed in curly braces {}.
● The tuple fields do not have to be homogeneous.
● Tuple fields can be extracted using pattern matching.
● Atom _ matches anything.

ExamplesExamples
1> Food = {{apples, 12}, {oranges, 32}}.
{{apples,12},{oranges,32}}
2> {_,{_,Noranges}} = Food.
{{apples,12}, {oranges,32}}
3> NOranges.
32

ListsLists
● A list is an ordered collection of values enclosed in square brackets [].
● A list has a head H (car) and a tail T (cdr). T must be a list (can be an
empty list []).
● Vertical bar | separates H from T: [ H | T ]
● List elements can be extracted by pattern matching.
● Always build lists by adding a head to a tail! Operator ++ is very
slow.

ExamplesExamples
1> Food = [{apples,12},{oranges,77},{foobar,3}].
[{apples,12},{oranges,77},{foobar,3}]
2> [Breakfast|MoreFood] = Food.
[{apples,12},{oranges,77},{foobar,3}]
3> Breakfast.
{apples,12}
4> {FruitForBreakfast,_} = Breakfast.
{apples,12}
5> FruitForBreakfast.
apples

Other OperatorsOther Operators
● =:= tests for equality
● Boolean operators: and, or, not, xor
● Short-circuit Boolean: orelse, andalso
● ++ (infix) appends lists (should be avoided)
● -- subtracts a list from a list

StringsStrings
● A string is a list of integers.
● A string can be also represented as a collection of printable symbols
enclosed into double quotes.
● Operator $ translates a character into its numerical value: $a is 97.
● [$h,$e,$l,$l,$o] is “hello.”

ModulesModules
● A module (mymod) is a logically complete piece of code in a file
(mymod.erl).
● Function c() compiles a module into bytecode: c(mymod). produces
file mymod.beam.
● Modules have attributes.
● Attribute -module(mymod). starts the module code.
● Attribute -import(othermod). imports a module.
● Attribute -export([list of funcs]). exports functions.
● Attributes must precede any other code.

FunctionsFunctions
● A function consists of one or more clauses with heads and bodies.
● The clauses are separated by a semicolon.
● The order of clauses matters.
● Clauses can be recursive.
● Functions must be declared in modules (not in the shell).
● Functions are polymorphic.

ExamplesExamples
-export[area/1].
area ({rectangle, H, W}) → W * H;
area ({circle, R}) → 3.14159 * R * R.
%% Another file!
-export[reduce/1,reduce/0].
reduce ([]) → 0;
reduce ([Head|Tail]) → someFuncOf (Head)
+ reduce (Tail).
%% Functions can be polymorphic.
reduce () → reduce ([]).

Anonymous Functions (Funs)Anonymous Functions (Funs)
● A function is a first-order object.
● It can be anonymous (like a closure)—a fun.
● Funs can be passed as parameters.
● Funs can be returned.

ExamplesExamples
%% file apply.erl
-export([apply1/2]).
apply1 (Func, X) → Func(X).
%% shell
1> apply.apply1 (fun(X)→X*X end,77).
5929
2> Invert = fun(X) → bnot X end.
#Fun<erl_eval.6.13229925>
3> apply:apply1 (Invert, 77).
-78
4> Invert(16).
17

Module “lists”Module “lists”
● Implicitly imported.
● lists:map/2 applies a function to a list and returns a new list
● lists:filter/2 returns a list of elements for which a function returns true
● lists:seq/2 returns a list of sequential integer numbers in a range
● lists:reverse/1 reverses a list (highly optimized!)

ExamplesExamples
1> lists:filter (fun (X) → X div 2 =:= X-1 end,
[1,2,3,4,5,6,7]).
[1,2]
2> lists:map (fun (X) → X*X end, [1,2,3,4,5,6,7]).
[1,4,9,16,25,36,49]

List ComprehensionList Comprehension
● List comprehensions are expression to create lists by rules
● A comprehension consists of a constructor and qualifiers (one or
more)
● A qualifier is a filter (a predicate or a boolean expression) or a
generator of the form Pattern←List

ExamplesExamples
1> [X*X || X←[1,2,3,4,5,6,7,8]].
[1,4,9,16,25,36,49,64]
2> [X*X || X←lists:seq (1,8)].
[1,4,9,16,25,36,49,64]
%% qsort
qsort ([])→[];
qsort ([Pivot|T]) → qsort ([X || X ← T, X < Pivot] ++
[Pivot] ++ qsort ([X || X←T, X >= Pivot]).
%% permutations
perms ([]) → [[]];
perms (L) → [[H|T] || H ← L, T ← perms (L--[H])].

Block ExpressionsBlock Expressions
● Used when more than one expression is needed to produce the value
(like progn in Lisp)
● The value of the block is the value of the last expression:
begin
expr1,
expr2,
expr3
end

GuardsGuards
● Guard sequence is a series of guards separated by “;” (meaning “or”)
● Guard is a sequence of guard expressions separated by “,” (meaning
“and”)
● Guard expression is true, constant (=false), guard predicate (is_atom,
is_number, etc.), guard built-in function (abs, float, etc.), term
comparison (/=,==,=/=,=:=,etc.), arithmetic expression or (short-
circuit) boolean expression

ExamplesExamples
small(X) when X=:=0 orelse 1/abs(X) > 1000 → true;
small(X) → false;
max(X,Y) when X>Y → X;
max(X,Y) when true → Y. %% when true is optional
… when is_tuple(X), size(X)=:=6, abs(element(3,X))>5
element(4,X)=:=hd(L) …

RecordsRecords
● Records are tuples with named elements.
● They are declared with -record() attribute.
● They are usually stored in *.hrl files.
● Those files are included with the -include(). attribute or
with rr() shell command
● Records are instantiated with the # operator with or without
default values
● Records can be copied with changes
● Fields can be extracted by matching

ExamplesExamples
%% File “student.hrl”
-record(student,{name,age,class=freshman}).
%% File “main.erl”
-include(“student.hrl”).
BestStudent = #student{}.
SeniorStudent = #student{class=senior}.
AnotherSenior = BestStudent#student{age=17}.
#student{age=Age} = AnotherSenior.
Age. %% Will print 17

ExamplesExamples
clear_status (#todo{status=S,who=W})=R) →
R#todo{status=finished}.

Conditional statementsConditional statements
case Expression of
Pattern1 [when Guard1] → seq1;
Pattern2 [when Guard2] → seq2;
end
if
Guard1 → sequence1;
Guard2 → sequence2;
end
If there is no match, an exception is raised. Atom true matches everything.

ExamplesExamples
%% Split a lists into odd and even elements
odds_and_evens (L) →
odds_and_evens (L,[],[]).
odds_and_evens ([H|T],Odds,Evens) →
case (H rem 2) of
1 → odds_and_evens (T,[H|Odds],Evens);
0 → odds_and_evens (T, Odds, [H, Evens])
end
odds_and_evens ([], Odds, Evens) →
{lists:reverse (Odds), lists:reverse (Evens)}.

● There are three types of errors in Erlang:
● exit(Why) broadcasts {'EXIT',Pid,Why} to all linked processes
● throw(Why) throws an exception
● erlang:error(Why) terminates the current process immediately
Raising ExceptionsRaising Exceptions

Catching an ExceptionCatching an Exception
try FuncOrExprSeq
%% This part can be omitted
of
Pattern1 [when Guard1] → Expression1;
...
catch
%% Type can be throw (default), exit, error or _:_
ExType1: ExPattern1 [when ExGuard1] → ExExpr1;
...
after %% Guaranteed to execute!
AfterExpression
end

ExampleExample
try math:sqrt(-1)
catch
error: {badarith, _} → ....
end

Process DictionaryProcess Dictionary
● Each process has a private dictionary: an associative array
● Built-in functions (BIFs) to work with dictionaries:
● @spec put (Key, Value) → OldValue.
● @spec get (Key) → Value | undefined.
● @spec get_keys () → [Key].
● @spec get () → [{Key, Value}].
● @spec erase (Key) → Value.
● @spec erase() → [{Key, Value}].

MacrosMacros
● Defined using -define(). attribute.
● Called using ?.
● Parameters allowed.
● Predefined macros: ?FILE, ?MODULE, ?LINE.
● Other attributes: -ifdef()., -undef()., -ifndef()., -else., -endif.

ExamplesExamples
%% To be used in binaries
-define (DWORD, 32/unsigned-little-integer).
?DWORD.
%% Simple macro with parameters
-ifndef (isodd).
-define (isodd (X), X rem 2 =:= 1).
-endif.
?isodd (77).

This presentation roughly covers the first 27
pages (of the total of 29
) of
“Programming Erlang. Software for a Concurrent World,” by Joe
Armstrong.

Introduction to Erlang Part 1

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