% Percent sign starts a one-line comment. %% Two percent characters shall be used to comment functions. %%% Three percent characters shall be used to comment modules. % We use three types of punctuation in Erlang. % Commas (`,`) separate arguments in function calls, data constructors, and % patterns. % Periods (`.`) (followed by whitespace) separate entire functions and % expressions in the shell. % Semicolons (`;`) separate clauses. We find clauses in several contexts: % function definitions and in `case`, `if`, `try..catch`, and `receive` % expressions. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %% 1. Variables and pattern matching. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % In Erlang new variables are bound with an `=` statement. Num = 42 . % All variable names must start with an uppercase letter. % Erlang has single-assignment variables; if you try to assign a different % value to the variable `Num`, you’ll get an error. Num = 43 . % ** exception error: no match of right hand side value 43 % In most languages, `=` denotes an assignment statement. In Erlang, however, % `=` denotes a pattern-matching operation. When an empty variable is used on the % left hand side of the `=` operator to is bound (assigned), but when a bound % variable is used on the left hand side the following behaviour is observed. % `Lhs = Rhs` really means this: evaluate the right side (`Rhs`), and then % match the result against the pattern on the left side (`Lhs`). Num = 7 * 6 . % Floating-point number. Pi = 3 . 14159 . % Atoms are used to represent different non-numerical constant values. Atoms % start with lowercase letters, followed by a sequence of alphanumeric % characters or the underscore (`_`) or at (`@`) sign. Hello = hello . OtherNode = example @ node . % Atoms with non alphanumeric values can be written by enclosing the atoms % with apostrophes. AtomWithSpace = 'some atom with space' . % Tuples are similar to structs in C. Point = { point , 10 , 45 }. % If we want to extract some values from a tuple, we use the pattern-matching % operator `=`. { point , X , Y } = Point . % X = 10, Y = 45 % We can use `_` as a placeholder for variables that we’re not interested in. % The symbol `_` is called an anonymous variable. Unlike regular variables, % several occurrences of `_` in the same pattern don’t have to bind to the % same value. Person = { person , { name , { first , joe }, { last , armstrong }}, { footsize , 42 }}. {_, {_, {_, Who }, _}, _} = Person . % Who = joe % We create a list by enclosing the list elements in square brackets and % separating them with commas. % The individual elements of a list can be of any type. % The first element of a list is the head of the list. If you imagine removing % the head from the list, what’s left is called the tail of the list. ThingsToBuy = [{ apples , 10 }, { pears , 6 }, { milk , 3 }]. % If `T` is a list, then `[H|T]` is also a list, with head `H` and tail `T`. % The vertical bar (`|`) separates the head of a list from its tail. % `[]` is the empty list. % We can extract elements from a list with a pattern-matching operation. If we % have a nonempty list `L`, then the expression `[X|Y] = L`, where `X` and `Y` % are unbound variables, will extract the head of the list into `X` and the tail % of the list into `Y`. [ FirstThing | OtherThingsToBuy ] = ThingsToBuy . % FirstThing = {apples, 10} % OtherThingsToBuy = [{pears, 6}, {milk, 3}] % There are no strings in Erlang. Strings are really just lists of integers. % Strings are enclosed in double quotation marks (`"`). Name = "Hello" . [ 72 , 101 , 108 , 108 , 111 ] = "Hello" . %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %% 2. Sequential programming. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % Modules are the basic unit of code in Erlang. All the functions we write are % stored in modules. Modules are stored in files with `.erl` extensions. % Modules must be compiled before the code can be run. A compiled module has the % extension `.beam`. - module ( geometry ). - export ([ area / 1 ]). % the list of functions exported from the module. % The function `area` consists of two clauses. The clauses are separated by a % semicolon, and the final clause is terminated by dot-whitespace. % Each clause has a head and a body; the head consists of a function name % followed by a pattern (in parentheses), and the body consists of a sequence of % expressions, which are evaluated if the pattern in the head is successfully % matched against the calling arguments. The patterns are matched in the order % they appear in the function definition. area ({ rectangle , Width , Ht }) -> Width * Ht ; area ({ circle , R }) -> 3 . 14159 * R * R . % Compile the code in the file geometry.erl. c ( geometry ). % {ok,geometry} % We need to include the module name together with the function name in order to % identify exactly which function we want to call. geometry : area ({ rectangle , 10 , 5 }). % 50 geometry : area ({ circle , 1 . 4 }). % 6.15752 % In Erlang, two functions with the same name and different arity (number of % arguments) in the same module represent entirely different functions. - module ( lib_misc ). - export ([ sum / 1 ]). % export function `sum` of arity 1 % accepting one argument: list of integers. sum ( L ) -> sum ( L , 0 ). sum ([], N ) -> N ; sum ([ H | T ], N ) -> sum ( T , H + N ). % Funs are "anonymous" functions. They are called this way because they have % no name. However, they can be assigned to variables. Double = fun ( X ) -> 2 * X end . % `Double` points to an anonymous function % with handle: #Fun<erl_eval.6.17052888> Double ( 2 ). % 4 % Functions accept funs as their arguments and can return funs. Mult = fun ( Times ) -> ( fun ( X ) -> X * Times end ) end . Triple = Mult ( 3 ). Triple ( 5 ). % 15 % List comprehensions are expressions that create lists without having to use % funs, maps, or filters. % The notation `[F(X) || X <- L]` means "the list of `F(X)` where `X` is taken % from the list `L`." L = [ 1 , 2 , 3 , 4 , 5 ]. [ 2 * X || X <- L ]. % [2,4,6,8,10] % A list comprehension can have generators and filters, which select subset of % the generated values. EvenNumbers = [ N || N <- [ 1 , 2 , 3 , 4 ], N rem 2 == 0 ]. % [2, 4] % Guards are constructs that we can use to increase the power of pattern % matching. Using guards, we can perform simple tests and comparisons on the % variables in a pattern. % You can use guards in the heads of function definitions where they are % introduced by the `when` keyword, or you can use them at any place in the % language where an expression is allowed. max ( X , Y ) when X > Y -> X ; max ( X , Y ) -> Y . % A guard is a series of guard expressions, separated by commas (`,`). % The guard `GuardExpr1, GuardExpr2, ..., GuardExprN` is true if all the guard % expressions `GuardExpr1`, `GuardExpr2`, ..., `GuardExprN` evaluate to `true`. is_cat ( A ) when is_atom ( A ), A =:= cat -> true ; is_cat ( A ) -> false . is_dog ( A ) when is_atom ( A ), A =:= dog -> true ; is_dog ( A ) -> false . % We won't dwell on the `=:=` operator here; just be aware that it is used to % check whether two Erlang expressions have the same value *and* the same type. % Contrast this behaviour to that of the `==` operator: 1 + 2 =:= 3 . % true 1 + 2 =:= 3 . 0 . % false 1 + 2 == 3 . 0 . % true % A guard sequence is either a single guard or a series of guards, separated % by semicolons (`;`). The guard sequence `G1; G2; ...; Gn` is true if at % least one of the guards `G1`, `G2`, ..., `Gn` evaluates to `true`. is_pet ( A ) when is_atom ( A ), ( A =:= dog );( A =:= cat ) -> true ; is_pet ( A ) -> false . % Warning: not all valid Erlang expressions can be used as guard expressions; % in particular, our `is_cat` and `is_dog` functions cannot be used within the % guard sequence in `is_pet`'s definition. For a description of the % expressions allowed in guard sequences, refer to the specific section % in the Erlang reference manual: % http://erlang.org/doc/reference_manual/expressions.html#guards % Records provide a method for associating a name with a particular element in a % tuple. % Record definitions can be included in Erlang source code files or put in files % with the extension `.hrl`, which are then included by Erlang source code % files. - record ( todo , { status = reminder , % Default value who = joe , text }). % We have to read the record definitions into the shell before we can define a % record. We use the shell function `rr` (short for read records) to do this. rr ( "records.hrl" ). % [todo] % Creating and updating records: X = #todo {}. % #todo{status = reminder, who = joe, text = undefined} X1 = #todo { status = urgent , text = "Fix errata in book" }. % #todo{status = urgent, who = joe, text = "Fix errata in book"} X2 = X1 #todo { status = done }. % #todo{status = done, who = joe, text = "Fix errata in book"} % `case` expressions. % `filter` returns a list of all elements `X` in a list `L` for which `P(X)` is % true. filter ( P , [ H | T ]) -> case P ( H ) of true -> [ H | filter ( P , T )]; false -> filter ( P , T ) end ; filter ( P , []) -> []. filter ( fun ( X ) -> X rem 2 == 0 end , [ 1 , 2 , 3 , 4 ]). % [2, 4] % `if` expressions. max ( X , Y ) -> if X > Y -> X ; X < Y -> Y ; true -> nil end . % Warning: at least one of the guards in the `if` expression must evaluate to % `true`; otherwise, an exception will be raised. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %% 3. Exceptions. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % Exceptions are raised by the system when internal errors are encountered or % explicitly in code by calling `throw(Exception)`, `exit(Exception)`, or % `erlang:error(Exception)`. generate_exception ( 1 ) -> a ; generate_exception ( 2 ) -> throw ( a ); generate_exception ( 3 ) -> exit ( a ); generate_exception ( 4 ) -> { 'EXIT' , a }; generate_exception ( 5 ) -> erlang : error ( a ). % Erlang has two methods of catching an exception. One is to enclose the call to % the function that raises the exception within a `try...catch` expression. catcher ( N ) -> try generate_exception ( N ) of Val -> { N , normal , Val } catch throw : X -> { N , caught , thrown , X }; exit : X -> { N , caught , exited , X }; error : X -> { N , caught , error , X } end . % The other is to enclose the call in a `catch` expression. When you catch an % exception, it is converted into a tuple that describes the error. catcher ( N ) -> catch generate_exception ( N ). %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %% 4. Concurrency %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % Erlang relies on the actor model for concurrency. All we need to write % concurrent programs in Erlang are three primitives: spawning processes, % sending messages and receiving messages. % To start a new process, we use the `spawn` function, which takes a function % as argument. F = fun () -> 2 + 2 end . % #Fun<erl_eval.20.67289768> spawn ( F ). % <0.44.0> % `spawn` returns a pid (process identifier); you can use this pid to send % messages to the process. To do message passing, we use the `!` operator. % For all of this to be useful, we need to be able to receive messages. This is % achieved with the `receive` mechanism: - module ( calculateGeometry ). - compile ( export_all ). calculateArea () -> receive { rectangle , W , H } -> W * H ; { circle , R } -> 3 . 14 * R * R ; _ -> io : format ( "We can only calculate area of rectangles or circles." ) end . % Compile the module and create a process that evaluates `calculateArea` in the % shell. c ( calculateGeometry ). CalculateArea = spawn ( calculateGeometry , calculateArea , []). CalculateArea ! { circle , 2 }. % 12.56000000000000049738 % The shell is also a process; you can use `self` to get the current pid. self (). % <0.41.0> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %% 5. Testing with EUnit %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % Unit tests can be written using EUnits's test generators and assert macros - module ( fib ). - export ([ fib / 1 ]). - include_lib ( "eunit/include/eunit.hrl" ). fib ( 0 ) -> 1 ; fib ( 1 ) -> 1 ; fib ( N ) when N > 1 -> fib ( N - 1 ) + fib ( N - 2 ). fib_test_ () -> [ ? _ assert ( fib ( 0 ) =:= 1 ), ? _ assert ( fib ( 1 ) =:= 1 ), ? _ assert ( fib ( 2 ) =:= 2 ), ? _ assert ( fib ( 3 ) =:= 3 ), ? _ assert ( fib ( 4 ) =:= 5 ), ? _ assert ( fib ( 5 ) =:= 8 ), ? _ assertException ( error , function_clause , fib ( - 1 )), ? _ assert ( fib ( 31 ) =:= 2178309 ) ]. % EUnit will automatically export to a test() function to allow running the tests % in the erlang shell fib : test () % The popular erlang build tool Rebar is also compatible with EUnit % ``` % rebar eunit % ```