Practical Erlang Programming Basic Erlang

© Erlang Training and Consulting Ltd2Basic Erlang Practical Erlang Programming

© Erlang Training and Consulting Ltd3Basic Erlang Overview: Basic Erlang I Basic Erlang I  Data Types  Variables  Complex Data Structures  Pattern Matching Basic Erlang II

© Erlang Training and Consulting Ltd4Basic Erlang Data Types: Integers #AB10F 2# $A B#Val is used to store numbers in base $Char is used for ascii values  example $A instead of 65 Large integers are converted to bignums Max size depends on physical constraints  RAM  paging memory

© Erlang Training and Consulting Ltd5Basic Erlang Data Types: Floats E-10 Not efficiently implemented Stored as a double  64-bit representation Follows the IEEE 754 standard

© Erlang Training and Consulting Ltd6Basic Erlang Data Types: Atoms person start_with_a_lower_case_letter 'Blanks can be quoted' 'Anything inside \n\012' Atoms are constant literals Start with a lower case letter or are encapsulated by ’ ’ Any character code is allowed within an atom if using ’ ’ Letters integers and _ are allowed if the atom starts with a lower case letter

© Erlang Training and Consulting Ltd7Basic Erlang Data Types: Tuples {123, bcd} {123, def, abc} {person, 'Joe', 'Armstrong'} {abc, {def, 123}, jkl} {} Tuples are used to store a fixed number of items Tuples of any size are allowed Contain valid Erlang expressions

© Erlang Training and Consulting Ltd8Basic Erlang Data Types: Lists [1, 2, 3, 4, 5, 6, 7, eight, nine] Used to store a variable number of items Lists are dynamically sized Strings in Erlang are lists of ASCII values Lists are written beginning with a [ and ending with a ] Elements are separated by commas

© Erlang Training and Consulting Ltd9Basic Erlang Data Types: Lists [123, xyz] [123, def, abc] length([a,[b,[c,d,e],f], g]) == 3. [{person, 'Joe', 'Armstrong'}, {person, 'Robert', 'Virding'}, {person, 'Mike', 'Williams'} ] "abcdefghi” == [97,98,99,100,101,102,103,104,105] ”” == [] [1 | [2 | [3 | [4 | []]]]] == [1,2,3|[4]] == [1,2,3,4]

© Erlang Training and Consulting Ltd10Basic Erlang Data Types: Lists List =[Element | List] or [] A recursive list definition consists of a head and a tail Lists whose last Tail term is [] are called:  proper lists or  well formed lists The Tail can be any valid Erlang datatype Most Erlang programs manipulate proper lists

© Erlang Training and Consulting Ltd11Basic Erlang Data Types: Lists String Concatenation “ABC” “DEF” == “ABCDEF” List Concatenation [1,2,3] ++ [4,5,6] == [1,2,3,4,5,6] List Subsets [1,2,3,4] -- [2,3] == [1,4]

© Erlang Training and Consulting Ltd12Basic Erlang Variables Abc A_long_variable_name AnObjectOrientatedVariableName _a_do_not_care_variable _ Variables start with an Upper Case Letter or _ They may not contain any “funny characters” _ alone is a don’t care variable  Its values are ignored and never bound

© Erlang Training and Consulting Ltd13Basic Erlang Variables Variables are used to store values of data structures The value of a variable can not be changed once it has been bound There is no need to declare them. Just use them! Erlang does not have a type system Types are determined at run time Variables can only be bound once!

© Erlang Training and Consulting Ltd14Basic Erlang Complex Data Structures [{{person,'Joe', 'Armstrong'}, [{telephoneNumber, [3,5,9,7]}, {shoeSize, 42}, {pets, [{cat, tubby},{cat, tiger}]}, {children,[{thomas, 5},{claire,1}]}] }, {{person,'Mike','Williams'}, [{shoeSize,41}, {likes,[boats, beer]}] } ].

© Erlang Training and Consulting Ltd15Basic Erlang Complex Data Structures Arbitrary complex structures can be created by nesting other data structures Data structures may contain bound variables Data structures are created by writing them down No explicit memory allocation or deallocation is needed  Allocated automatically  Deallocated by the Garbage Collector when no longer referenced

© Erlang Training and Consulting Ltd16Basic Erlang Pattern = Expression Pattern Matching Pattern Matching is used for: Assigning values to Variables Controlling the execution flow of the programs Extracting values from compound data types

© Erlang Training and Consulting Ltd17Basic Erlang Pattern Matching: Assigning The Pattern can contain unbound variables which are bound when the pattern matching succeeds The Expression may not contain unbound variables A = 10 Succeeds: binds A to 10 {B, C, D} = {10, foo, bar} Succeeds: binds B to 10, C to foo and D to bar {E, E, foo} = {abc, abc, foo} Succeeds: binds E to abc [H|T] = [1,2,3] Succeeds: binds H to 1, T to [2,3]

© Erlang Training and Consulting Ltd18Basic Erlang Pattern Matching: Testing A match must either succeed or fail Used to pick the execution flow in:  case statements  receive statements  function heads {A, A, B} = {abc, def, 123} Fails [A,B,C,D] = [1,2,3] Fails [A,B|C] = [1,2,3,4,5,6,7] Succeeds: binds A = 1, B = 2, C = [3,4,5,6,7] [H|T] = [] Fails

© Erlang Training and Consulting Ltd19Basic Erlang Pattern Matching: Extracting Used to extract values from complex data types Note the use of _, the don't care variable {A,_, [B|_],{B}} = {abc,23,[22,x],{22}} Succeeds: binds A = abc, B = 22 C = 10, {C, C, 13, D, _} = {10,10,13,12,15} Succeeds: Binds D = 12 and C = 10 Var = {person, ‘Francesco’, ‘Cesarini’}, {person, Name, Surname} = Var Succeeds: binds Name = ‘Francesco’, Surname = ‘Cesarini’ [Element|Tail] = [1,2,3,4] Succeeds: binds Element = 1, Tail = [2,3,4]

© Erlang Training and Consulting Ltd20Basic Erlang Summary: Basic Erlang I Basic Erlang I  Data Types  Variables  Complex Data Structures  Pattern Matching Basic Erlang II

© Erlang Training and Consulting Ltd21Basic Erlang Overview: Basic Erlang II Basic Erlang I Basic Erlang II  Function Calls  Modules  Starting the System

© Erlang Training and Consulting Ltd22Basic Erlang Functions: Syntax area({square, Side}) -> Side * Side ; area({circle, Radius}) -> 3.14 * Radius * Radius; area({triangle, A, B, C}) -> S = (A + B + C)/2, math:sqrt(S*(S-A)*(S-B)*(S-C)) ; area(Other) -> {error, invalid_object}. function clauses function head expression separator function separator clause separator ; ; function body

© Erlang Training and Consulting Ltd23Basic Erlang Functions: Calls Module:Function(Arg1, Arg2,... ArgN) Function(Arg1, Arg2,.. ArgN) Erlang programs consist of functions that call each other Functions are defined within Modules Function and module names must be atoms The arity of a function is its number of arguments

© Erlang Training and Consulting Ltd24Basic Erlang Functions: Syntax Func(Pattern1, Pattern2,...) ->, …… ; Func(Pattern1, Pattern2,...) ->, …… ; …… Func(Pattern1, Pattern2,...) ->, ……. A function is defined as a collection of clauses Variables are pattern matched in the function head If pattern matching fails on a clause, the next one is tested  One clause must always succeed Functions return value of the last expression executed

© Erlang Training and Consulting Ltd25Basic Erlang factorial(0) -> 1; factorial(N) -> N * factorial(N-1). > factorial(3). (matches N = 3 in clause 2) == 3 * factorial(3 - 1) (matches clause 2) == 3 * 2 * factorial(2 - 1) (matches clause 2) == 3 * 2 * 1 * factorial(1 - 1) (matches clause 1) == 3 * 2 * 1 * 1 = 6 Pattern Matching occurs in the function head  Unbound variables get bound after a successful pattern match Variables are local to each clause Variables are allocated and deallocated automatically Functions: Examples

© Erlang Training and Consulting Ltd26Basic Erlang Modules Modules are stored in files with the.erl suffix The module and file names must be the same  Modules are named with the -module(Name). directive Exported functions can be called from outside the module  Export them with  -export([Function/Arity, Function/Arity, …])  Use the module prefix when making the call Local functions may only be called within the module

© Erlang Training and Consulting Ltd27Basic Erlang -module(demo). -export([double/1]). % This is a comment. % Everything after % is ignored! double(X) -> times(X, 2). times(X, N) -> X * N. Modules module directivesmodule name exported functions functions arity

© Erlang Training and Consulting Ltd28Basic Erlang Starting the System Start the Erlang system by typing erl in the unix shell, or double clocking on the Erlang icon in Windows. An Erlang shell will be started. 1>, 2> … are shell prompts. The shell is an Erlang process that sits in a read-eval-print loop. It reads valid Erlang expressions typed in by the user and evaluates them. c(File) compiles the file File.erl and loads the module in the shell. Ensure the current shell directory is in the same as the location of the module you are compiling.  Use cd(“Path”) and pwd() In the shell.

© Erlang Training and Consulting Ltd29Basic Erlang Eshell V5.0.1 (abort with ^G) 1> c(demo). {ok, demo} 2> demo:double(25). 50 3> demo:times(4,3). ** exited: {undef,{demo,times,[4,3]}} ** 4> >

© Erlang Training and Consulting Ltd30Basic Erlang Exercise Write a module boolean.erl that takes logical expressions and boolean values (represented as the atoms true and false) and returns their boolean result. The functions you should write should include b_not/1, b_and/2, b_or/2, b_nand/2. You may not use the logical constructs and, or or not. Test your module from the shell. Interface: b_not(false)  true b_and(false, true)  false b_and(b_not(b_and(true, false)), true)  true Note 1: foo(X)  Y means that calling the function foo with the parameter X will result in the value Y being returned. Note 2: and, or and not are reserved words in Erlang.

© Erlang Training and Consulting Ltd31Basic Erlang Summary: Basic Erlang II Basic Erlang I Basic Erlang II  Function Calls  Modules  Starting the System