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 = 7.72> Apples = 12.123> Apples1 = Apples / 5 + Oranges.9.44> Apples = Apples.125> 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 unbound2> apple. apple3> '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).%% shell1> apply.apply1 (fun(X)→X*X end,77). 59292> Invert = fun(X) → bnot X end. #Fun<erl_eval.6.13229925>3> apply:apply1 (Invert, 77). -784> 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]

%% qsortqsort ([])→[];qsort ([Pivot|T]) → qsort ([X || X ← T, X < Pivot] ++

[Pivot] ++ qsort ([X || X←T, X >= Pivot]).

%% permutationsperms ([]) → [[]];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))>5element(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 ofPattern1 [when Guard1] → seq1;Pattern2 [when Guard2] → seq2;

end

ifGuard1 → 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 elementsodds_and_evens (L) →

odds_and_evens (L,[],[]).odds_and_evens ([H|T],Odds,Evens) →

case (H rem 2) of1 → odds_and_evens (T,[H|Odds],Evens);0 → odds_and_evens (T, Odds, [H, Evens])

endodds_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 omittedofPattern1 [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.