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README.md

Elixir

Elixir is a programming language built on top of Erlang. As Erlang, it is a functional language with strict evaluation, single assignment and dynamic typing built to support distributed, fault-tolerant, non-stop applications with hot swapping. Elixir allows you to invoke Erlang modules without a need to convert data types, therefore there is no hit in performance when invoking existing Erlang code.

The main difference between Elixir and Erlang is its syntax inspired by Ruby and method dispatching and metaprogramming on top of modules.

Usage

Elixir is still in development. If you want to help building it or are just looking for some fun, you can get started now! First, you need to clone this repository to your machine, compile and test it:

$ git clone https://github.com/josevalim/elixir.git
$ cd elixir
$ make test

$ bin/elixir -v
Elixir 0.3.0

If tests fail, it is likely you have an outdated Erlang version. You can check your Erlang version by calling erl in the command line. You will see some information as follow:

Erlang R14B01 (erts-5.8.2) [source] [64-bit] [smp:2:2] [rq:2] [async-threads:0] [hipe] [kernel-poll:false]

Elixir requires Erlang R14B01 or later version to execute (R14A and R14B do not work). If you have the correct version and tests still fail, feel free to open an issue in the issues tracker on Github. If all tests pass, you are ready to play with Elixir!

This README provides a length explanation about Elixir in the Learning Elixir section below. There are also some examples in the examples folder that you can run by executing the bin/elixir EXAMPLE and an interactive Elixir available as bin/iex. Feel free to build your own examples and study the language better.

Contributing & Roadmap

Currently, there is an effort to improve Elixir Standard Library. As much of Elixir's STDLIB is written in Elixir and tested in Elixir, you don't need to be an advanced Erlang user to improve the language, just know the OTP a bit. As an example, you may take a look at the List implementation and its tests to check how simple it is.

If you want to contribute to Elixir, the code is organized as follow:

  • include, src - Both directories contain the part of the source code written in Erlang. leex and yecc were used as tokenizer and parser respectively;

  • lib - Contains Elixir's STDLIB, written in Elixir;

  • test/elixir - Tests for Elixir's STDLIB, written in Elixir. For this purpose, Elixir ships with a small unit test library called ExUnit;

  • test/erlang - Contains tests for Elixir, written in Erlang. Usually, just internal stuff is tested here. The preferred way to test is in Elixir itself.

If you are interested, check out the ROADMAP.md file in the repository or keep reading this README to find items to be implemented.

Important links

Learning Elixir

This is a basic introduction into Elixir. Some sections have a paragraph called "To be implemented", they represent parts of Elixir that was not implemented yet and that are under discussion.

This introduction borrowed its guidelines from Learn You Some Erlang, a great resource to learn Erlang which will be referenced several times during this introduction.

Hello World

Let's start with a simple hello world. The first step is to create a new file called "hello.ex" inside Elixir repository with the following contents:

module Hello
  def world
    IO.puts "Hello World"
  end
end

Now, we can compile this file to the current directory:

bin/elixirc hello.ex -o .

Notice that a .beam file was added to the current directory with the compiled code. We can execute it by invoking the method world in the module Hello in the same directory:

bin/elixir -e "Hello.world"

And you will see "Hello World" printed! This example works because Elixir automatically loads the compiled files in the current directory. If your compiled files are in other directories, you can pass those new directories to bin/elixir using -pa and -pz as options. Type bin/elixir with no arguments for more information.

When you are building libraries in Elixir, those are the main steps you should take. Write your code, compile it and run it! However, sometimes it is nice to just put some code together and run it, without a explicit compilation step. For that, elixir allows you to easily create scripts. Let's create a new file "hello.exs" with the following contents:

IO.puts "Hello World"

And now run it:

bin/elixir hello.exs

And it works again! Notice we used the extension .exs instead of .ex here. This is just a convention, Elixir does not treat .exs files differently from .ex files in any way! In fact, you could even try to compile the .exs file:

bin/elixirc hello.exs -o .

When you do that, you can see that "Hello World" is printed as well. This is because Elixir actually executes the files to compile them. This is the key to many Elixir features, as we are going to see later.

Also notice that Elixir ships with an interactive console that you can use for most examples in this tutorial, you can start it with:

bin/iex

Enjoy!

Some notation

Before we start, notice that comments in Elixir are, as in Erlang, done with %.

% This is a commented line

Throughout this introduction, % => represents the result of an expression:

1 + 1 % => 2

Basic Types

Numbers

Elixir supports both Integer and Floats:

2 + 15       % => 17
- 13 * 10    % => -130
1986 - 1985  % => 1
5 / 2        % => 2.5
4 / 2        % => 2.0

Notice that, as Erlang, "/" always returns a Float. If you want to have integer-to-integer division and the modulo operator, you should use div and rem:

5 div 2  % => 2
5 rem 2  % => 1

Several operations can also be done in a single expression, obeying the normal precedence rules:

50 * 10 - 490     % => 10
(50 * 10) - 490   % => 10
-(50 * 10) - 490  % => -990

In Elixir, you can dispatch methods to data types, this is further explained later:

-1.abs    % => 1
5.div(2)  % => 2

It comes as no surprise that + is also a method:

1.+(2)  % => 3

Finally, notice that Elixir allows you to include "_" in numbers (as in Ruby). This improves the readability when working with large numbers:

1_052_672

Documentation:

To be implemented

Currently, there is no support to enter numbers in bases other than base 10. This is the current API in Erlang (although the best API for Elixir is under discussion):

2#101010.  % => 42
8#0677.    % => 447
16#AE.     % => 174

Atoms

Elixir also has Atoms, called Symbols in other languages like Ruby. Although its syntax was borrowed from Lisp:

'atom
'Atom
'atom_without_spaces

Atoms are literals, with their own value as name. An atom 'symbol is an atom 'symbol everywhere, with exactly the same value. Atoms start with a single quote and should not have spaces (spaces delimit the atom end). Atoms with spaces are represented by wrapping them in quotes:

'"Atom with Spaces"

As in Erlang and Ruby, Atoms are not garbage collected, so remember to not generate atoms dynamically, otherwise you will run out of memory sooner rather than later.

Documentation:

Booleans

As in Erlang, the boolean values are simply atoms named true and false. However, to avoid writing 'true and 'false, Elixir also allows you to simply write true or false. The following are all equivalent and will yield 1 as result:

if 'true
  1
else
  2
end

if true
  1
else
  2
end

if 'false
  2
else
  1
end

if false
  2
else
  1
end

Besides those two boolean values, Elixir also has a nil value which is simply an atom as well. nil also evaluates to false in conditionals.

Tuples

Tuples are used to organize many terms together when you know how many terms there are. As in Erlang, a tuple is written in the following form:

% A tuple containing all boolean values
{ true, false }

% A tuple that may represent a point with coordinates X and Y
{ 10, 20 }

% An empty tuple
Tuple.new

Tuples and lists (which are going to see next), are zero-indexed in Elixir while they are one-indexed in Erlang. You can retrieve a specific element using []:

{'a,'b,'c}[1]  % => 'b
{'a,'b,'c}[2]  % => 'c

Documentation:

Lists

Lists are the main structure in Elixir (as in any other functional language) and can contain anything:

% Some list with elements
['atom, 1, 2, 3, { 'some, 'tuple }]

% An empty list
[]

Elixir Standard Library has a bunch of methods to interact with lists:

[1, 2, 3].length       % => 3
['a, 'b, 'c][1]        % => 'b

As in Elixir + is simply a method like any other (and not an arithmetic operator as in Erlang), it can also be used to add arrays:

[1, 2, 3] + [4, 5, 6]  % => [1,2,3,4,5,6]

Lists in Erlang and Elixir are implemented as linked lists. This means prepending an item to the list is quite fast, but appending is much slower. Therefore we have a special syntax to prepend one or more items to a list:

list = [2,3,4]

% Don't do this:
[1]   + [2,3,4]  % => [1,2,3,4]
[0,1] + [2,3,4]  % => [0,1,2,3,4]

% Do this instead:
[1|list]    % => [1,2,3,4]
[0,1|list]  % => [0,1,2,3,4]

Most of the power in lists comes when used together with functions:

[1, 2, 3].map do (x)
  x * 2
end  % => [2, 4, 6]

[1, 2, 3].foldl 0, do (x, acc)
  acc + x
end  % => 6

The examples above uses functions using the do/end syntax. Don't worry about them now, we are going to take a better look at them later.

Ordered Dicts

Elixir provides a first-class syntax to deal with ordered dictionaries (similar to Hashes in Ruby).

% A dict with 'a and 'b as keys and 1 and 2 as their respective values
{ 'a: 1, 'b: 2 }

% An empty dict
{}

Elixir dictionary implementation is backed up by the orddict module in OTP. Notice that Erlang ordered dicts are not ordered in the order items are added, but rather using Erlang ordering of terms. You can learn more about Erlang ordering by reading this section from Learn You Some Erlang.

Ordered Dicts are recommended to deal with small amount of data. Other data structures are recommended to deal with a huge amount and you can read more about others key-value store, but remember that most of them are not implemented in Elixir yet.

Documentation:

Bit strings

Elixir has a similar syntax to Erlang for handling bit strings:

% A bit string with three elements
<<1, 17, 42>>

% Converting a bit string to a list
<<1, 17, 42>>.to_list  % => [1, 17, 42]

Elixir also allows to specify the size for bit strings, using the same syntax as Erlang:

% A bit string with size 4, because we specify that 42 is a 16-bits segment
<<1, 17, 42:16>>

By default, the bit string type in both Elixir and Erlang is integer. That said, the following is invalid:

<<3.14>>

Instead, you need explicitly specify it as a float:

<<3.14|float>>

Notice the syntax above is a bit different from Erlang. Erlang uses / to specify the type, Elixir uses |. This allows Elixir, differently from Erlang, to have expressions inside bit string:

<<1+2>>

In general, everything that applies to Erlang bit string applies to Elixir bit string. You can read more about them on Erlang's documentation.

Documentation

Strings

In Erlang, strings are a list of chars:

"hello" == [104, 101, 108, 108, 111]

This is expensive because each character uses 8 bytes of memory, not 8 bits! Erlang stores each character as a 32-bit integer, with a 32-bit pointer for the next item in the list.

Elixir takes a different approach to strings. Strings in Elixir are handled as UTF-8 binaries. Since a binary is nothing more than a bit string, where the number of bits is a multiple of 8, we can create strings using the bit string syntax:

<<72, 73, 74>>  % => "HIJ"

When a bit string with multiple of 8 bits is created, it is automatically mapped to a string. However, you will rarely use the syntax above as Elixir provides the more traditional quote syntax to handle strings:

% The famous "hello world" string
"hello world"

% A string converted to a char list:
"hello".to_char_list  % => [104, 101, 108, 108, 111]

% Convert a char list back to a binary/string:
[104, 101, 108, 108, 111].to_bin % => "hello"

% Notice that to_s in a list is not the same as to_bin.
% It returns the list represented as a string instead:
[104, 101, 108, 108, 111].to_s % => "[104,101,108,108,111]"

% Strings are UTF-8
"Arrow ⇧ up".length  % => 10

Keep in mind that, as Elixir strings are different from Erlang strings, sometimes you may need to convert Elixir strings to a char list and vice-versa when invoking Erlang methods using the methods to_char_list and to_bin as seen above.

Finally, strings also support interpolation:

"string #{'with} interpolation"  % => "string with interpolation"
"1 + 1 = #{1 + 1}"               % => "1 + 1 = 2"

Documentation

Functions

Functions are an important aspect of Elixir as in any functional programming language. Functions are created in Elixir with the keywords -> or do:

my_function = do
  1 + 2
end

my_function.call() % => 3

another_function = ->
  1 * 2
end

another_function.call() % => 2

Some functions expect arguments:

my_function = do (x, y)
  x + y
end

my_function.call(1, 2) % => 3

another_function = -> (x, y)
  x * y
end

another_function.call(1, 2) % => 2

You can also represent functions in one line, without a need for the closing keyword end:

my_function = do (x, y) x + y
my_function.call(1, 2) % => 3

another_function = -> (x, y) x * y
another_function.call(1, 2) % => 2

Notice that, whenever using one-line functions, if you need parentheses inside the expression, you are required to give empty parentheses arguments, for example:

% This works as expected:
my_function = -> 1 + 2
my_function.call % => 3

% This won't work and it raises a syntax error
my_function = -> (1 + 2)

% This works as well:
my_function = -> () (1 + 2)

In the second case, it is ambiguous if the parentheses is part of the argument list or the function expressions. This is why you either need to remove parentheses (as in the first example) or add empty parentheses (as in the third example). This syntax quickly proves to be very convenient:

[1,2,3].map(-> (x) x * 2)   % => [2,4,6]

In the example above, we are calling .map passing a function as argument. If we remove the optional parentheses:

[1,2,3].map -> (x) x * 2   % => [2,4,6]

Other examples using the multiline syntax:

[1,2,3].foldl(0, do (x, acc)
  acc + x
end) % => 6

Again, removing the parentheses, improves readability:

[1,2,3].foldl 0, do (x, acc)
  acc + x
end % => 6

Elixir also provides a shortcut syntax to invoke functions that is usually faster as it skips method lookup:

my_function = -> (x, y) x + y
my_function.(1, 2) % => 3

In such cases, parentheses are always required.

Another useful extension Elixir adds to functions is the easy generation of anonymous functions. For instance, suppose you have a list of cars and you want to get their names. A way to do that would be:

cars.map -> (c) c.name

However, you can easily generate an anonymous functions that does the same:

cars.map _.name

Anonymous functions can also be generated with arguments, so the map expressions we saw above:

[1,2,3].map -> (x) x * 2   % => [2,4,6]

Could actually be rewritten as:

[1,2,3].map _.*(2)

Currently, functions do not support partial applications or pipes, but such features will be added down the road.

Documentation

Variables and Pattern Matching

Variables in Elixir work differently from Erlang. You can assign values to the same variable several times:

x = 1
x = 2

You can force a match to happen prefixing ~ to the variable name:

~x = 3  % => Raises a bad match error, because x was last bound to 2

In Erlang/Elixir terms, a variable that was not assigned yet is called unbound variable. Let's see more examples:

% Let's bound the variable x to 'foo
x = 'foo

% Now let's match a tuple with other tuple.
% Since x is already bound, we are comparing x with 'baz and it will fail:
{ ~x, y } = { 'baz, 'bar }

% In this case, we compare 'x with 'foo and it matches.
% Since y is unbound, we assign 'bar to it:
{ ~x, y } = { 'foo, 'bar }

x  % => 'foo
y  % => 'bar

For lists, we can use the same syntax to prepend an item on pattern matching, easily retrieving the head and tail:

[h|t] = [1,2,3]
h  % => 1
t  % => [2,3]

% Raises an error because h was already assigned to 1 and 1 does not match 2
[~h|t1] = [2,3,4]

Elixir will often complain if you bound a value to a variable but never use it. For instance, imagine that you want to get just the first element of a tuple with three items:

{x, y, z} = {1, 2, 3}

If you don't use the y and z variables, Elixir will show you some warnings. For this reason, you could use _ instead:

{x, _, _} = {1, 2, 3}

The variable _ is always unbound:

_ = 1
_   % => Raises that variable '_' is unbound

Sometimes having several occurrences of _ in the same expression is confusing, so you can do this instead:

{x, _y, _z} = {1, 2, 3}

The values 2 and 3 will be bound to the variables _y and _z, but Elixir won't complain if you eventually don't use them.

Keep in mind that the number of expressions allowed in pattern matching are limited. You cannot invoke methods, use interpolated strings, retrieve constants and so on. Therefore, this is invalid:

1.abs = -1

Ordered dicts are also allowed in pattern matching but there is one important restriction: you are responsible to make their order match. Therefore, this won't work:

dict = { 2: 4, 1: 2 }
{ 2: 4, 1: 2 } = dict

This fails because the dict variable is ordered, so it is actually represented as {1: 2, 2: 4}. Remember that OrderedDicts are ordered according to Elixir ordering of terms and not the order new items are added. This ordering rule is important to allow us to bound variables to key-values:

dict = { 2: 4, 1: 2 }

% This matches as the left expression is in the correct order
{ 1: 2, 2: 4 } = dict

% This matches and bound x and y to 2 and 4
{ 1: 2, x: y } = dict

Method signatures

Finally, pattern matching can also be implemented in methods signatures. Here is the classic Fibonacci example:

module Math
  def fibonacci(0)
    0
  end

  def fibonacci(1)
    1
  end

  def fibonacci(n)
    fibonacci(n - 1) + fibonacci(n - 2)
  end
end

Math.fibonacci(0)   % => 0
Math.fibonacci(1)   % => 1
Math.fibonacci(3)   % => 2
Math.fibonacci(10)  % => 55

Notice the example above is not tail call optimized. We will discuss modules, methods and optimizations later.

Operators

Operators can be binary or unary operators:

(expression) binary_op (expression)
unary_op (expression)

Term comparisons

Elixir term comparisons operators are close to Erlang ones, except !=, =!= and <= which maps to Erlang /=, =/= and =<.

Operator Description
== equal to
!= not equal to
<= less than or equal to
< less than
>= greater than or equal to
> greater than
=:= exactly equal to
=!= exactly not equal to

As in Erlang, Elixir can order different types according to the following order:

number < atom < reference < fun < port < pid < tuple < modules < blank slates < list < bit string

Lists are compared element by element. Tuples are ordered by size, two tuples with the same size are compared element by element. If one of the compared terms is an integer and the other a float, the integer is first converted into a float, unless the operator is one of =:= and =!=.

All term comparison operators return a boolean expression.

Arithmetic operators

Operator Description Argument
+ unary + number
- unary - number
+ anything
- anything
* anything
/ returns a float anything
div returns an integer anything
rem returns an integer anything

Except by the two unary operators, all other operators can be overloaded. For instance, we can concatenate two lists by using the + operator:

[1,2,3] + [4,5,6]  % => [1,2,3,4,5,6]

This is the same as:

[1,2,3].+([4,5,6]) % => [1,2,3,4,5,6]

Notice however that we cannot add a list with a number:

[1,2,3] + 1  % => Raises an error

Also, Elixir keeps the same semantics as Erlang in the sense the / operator always returns a float when numbers are given as argument. The div and rem operators are used to deal with integers:

2 / 1    % => 2.0
6 div 4  % => 1
6 rem 4  % => 2

Bitwise operators

To be implemented/written.

Strict boolean operators

Elixir provides the following operators to deal strictly with booleans:

Operator Erlang equivalent Description
and and Both expressions must return boolean
or or Both expressions must return boolean
andalso andalso First expression must return boolean, short-circuit operator
orelse orelse First expression must return boolean, short-circuit operator
not not Unary operators, expression must be a boolean

Logical operators and control-flow

Elixir provides three operators general purposes operators:

Operator Description
&& and
|| or
! not

Remember that everything, except false and nil, evaluates to true:

!false       % => true
!true        % => false
!Object.new  % => false

Both && and || are actually control structures. They do not return a boolean but the last evaluated expression:

1 && 2       % => 2

true || false       % => true
'atom || 'another   % => 'atom
false || 'another   % => 'another

false && IO.puts("I will never be executed")

1 || IO.puts("I will never be executed")
true || IO.puts("I will never be executed")

Precedence

Operator precedence in falling priority:

Operator Associativity
. # Left
+ - ! not Non associative (unary operators)
/ * div rem Left
== != < <= > >= =:= =!= Left
<- Right
and andalso Left
or orelse Left
&& Left
|| Left

if/else and case/match

Elixir, differently from Erlang, has a more conventional if/else structure:

list = [1,2,3]

if list.include?(4)
  IO.puts "it includes 4"
elsif list.include?(5)
  IO.puts "it includes 5"
else
  IO.puts "it does not include 4 or 5"
end

Everything in Elixir, except false and nil, evaluates to true.

On the other hand, the case/match structure from Elixir is quite similar to Erlang's:

case {1,2,3}
match {3,2,x}
  x * 2
match {1,2,x}
  x * 2
end

As you can notice,case/match uses pattern matching. If no case expression matches, an error is raised. Elixir also allows an else clause in case/match, which is the same as match _:

case {4,5,6}
match {3,2,x}
  x * 2
match {1,2,x}
  x * 2
else
  10
end

Finally, case/match expressions can be inlined and grouped, providing a more compact syntax:

case {4,5,6}
match {3,2,x}, {1,2,x} then x * 2
else 10
end

Currently there is no support for guard expressions as in Erlang, although it may be implemented at some point.

Exceptions

Similarly to Erlang, Elixir has three kinds of exceptions. They are raised with the methods (and not keywords!) throw, error and exit. You can read more about each type on Learn You Some Erlang.

To handle these exceptions, Elixir uses a syntax similar to Ruby:

try
  self.throw {1,2}
catch {1,2}
  IO.puts "Rescued {1,2}"
end

Similar to the match syntax, you can catch different values in the same clause:

try
  self.throw {1,2}
catch {1,2}, {3,4}
  IO.puts "Rescued a tuple"
end

In order to catch an error or an exit, you need to be explicit:

try
  self.error {1,2}
catch {1,2}
  IO.puts "I will never get a tuple {1,2}"
catch 'error: {1,2}
  IO.puts "Rescue an error with {1,2}"
end

You must use the keyword after if you want to execute some code regardless if there was an exception or not:

try
  self.error {1,2}
catch {1,2}
  IO.puts "I will never get a tuple {1,2}"
after
  IO.puts "I am always executed"
end

It is important to keep in mind that tail calls are not optimized inside try blocks. This is expected as the runtime needs to keep the backtrace in case an exception occur. Also, notice that variables created inside try/catch/after clauses do not leak to the outer scope.

try
  foo = 13
end

foo % => raises undefined variable or local method foo error

try
  foo = 13
after
  IO.puts "I am always executed"
end

foo % => raises undefined variable or local method foo error

When used inside methods, the try/end can be omitted:

def some_method
  self.error {1,2}
catch {1,2}
  IO.puts "I will never get a tuple {1,2}"
after
  IO.puts "I am always executed"
end

Again, be careful when using this pattern with tail calls, as the try block is not optimized. For instance, consider this method:

def some_method([h|t], value)
  value = method_that_may_raise_error(h)
  some_method(t, value)
catch {1,2}
  IO.puts "I will never get a tuple {1,2}"
after
  IO.puts "I am always executed"
end

It should actually be written as:

def some_method([h|t], value)
  value = try
    method_that_may_raise_error(h)
  catch {1,2}
    IO.puts "I will never get a tuple {1,2}"
  after
    IO.puts "I am always executed"
  end

  some_method(t, value)
end

List of errors

Here is a list of runtime errors that can be raised by Elixir:

  • { 'builtin_not_allowed, { method, builtin } }

    Invoking method not allowed on the builtin structure. Builtins are all structures that comes directly from Erlang, they are: String, Integer, Float, Tuple, List, OrderedDict and so forth. Binding and setting instance variables are currently disabled on builtins;

  • { 'module_defined, { name, file, line } }

    An module with name was already defined on file at line. This is a common error to appear during compilation time as the following valid Ruby pattern is not valid in Elixir:

    module Foo
      module Bar
      end
    end
    
    module Foo
      module Baz
      end
    end
    

    In the example above, we are reopening Foo to add a Baz module. This is invalid in Elixir as modules cannot be reopened. To handle this, just define Baz directly as Foo::Baz:

    module Foo::Baz
    end
    
  • { 'no_local_method, { name, arity, module } }

    There isn't a local method with the given name and arity in module;

  • { 'not_a_module, { method, other } }

    method failed because other is not a module;

  • { 'no_module, name }

    A module with name could not be found;

  • { 'no_callback, { name, arity, structure } }

    The callback name with arity was not implemented in structure. Raised when a structure is given as callback but does not comply to all conditions;

  • { 'bad_ivar, name }

    The name given is not an atom and cannot be given as internal variable name;

  • { 'bad_binding, { module, actual } }

    Could not bind to module module as __bound__ callback returned actual;

  • { 'bad_ivars, value }

    value given to @() or set_ivars is not an OrderedDict or it is an OrderedDict but not all keys are atoms;

  • { 'internal_method_overridden, { method, arity } }

    The method with arity arity is defined automatically by Elixir and cannot be overridden.

Strings, Atoms, Regular Expressions, Interpolation and Sigils

In Elixir, we have the following basic types related to Strings:

% Strings (utf8 by default and represented as binaries)
"string"
"string #{'with} interpolation"    % => "string with interpolation"

% Integer representation of a character
$a    % => 97
$b    % => 98
$\\   % => 92
$\(   % => 40

% A string represented as a list of chars (all four expressions below allow interpolation)
$"string"    % => [115,116, 114, 105, 110, 103]
$(string)    % => [115,116, 114, 105, 110, 103]
$[string]    % => [115,116, 114, 105, 110, 103]
${string}    % => [115,116, 114, 105, 110, 103]

% A binary representing the list of chars above
<<115, 116, 114, 105, 110, 103>>

% Erlang Atoms or Ruby Symbols
'atom
'"atom with space and interpolation"
'(atom with space and interpolation)
'[atom with space and interpolation]
'{atom with space and interpolation}

Besides these basic types, we also have string sigils. Here is one example:

% Regular expressions
%% Without interpolation
~r(regexp)
~r[regexp]
~r{regexp}
~r"regexp"

%% With interpolation
%% It also accepts [], {} and "" as separators as above
~R(regexp #{1 + 1} interpolation)

%% With regexp operators
~r(foo)im

All string sigils follow the same set of rules. They start with a ~ followed by a letter and the string is delimited by a separator. The available separators are (), [], {} and "". If the letter after ~ is lowercased, no interpolation is allowed, if uppercased, interpolation is allowed. A couple more examples:

% Another way to create strings
~q(string without interpolation)
~Q{string without interpolation}

% Another way to create atoms
~a"atom without interpolation"
~A[atom with interpolation]

% Another way to create a list of chars
~l(string)  % => [115,116, 114, 105, 110, 103]
~L{string with interpolation}

% A list of words (to be implemented)
~w(foo bar baz)        % => ["foo", "bar", "baz"]
~W{foo #{'bar} baz}    % => ["foo", "bar", "baz"]

Documentation

Heredoc

Elixir also has HEREDOCs to make easier to handle big strings:

string = ~~
  This is a string which
  preserves whitespace at
  the beginning and also
  handles #{'interpolation}
~~

Similar to Ruby, HEREDOCs allow an identifier right after the initial three quotes:

string = ~~HTML
  <p>Nice!</p>
~~

This allows to identify the content and most text editor uses it to properly syntax highlight it. Besides, you can add Elixir code after the HEREDOC and they still are properly evaluated:

string = ~~STRING + "123"
abc
~~

string % => "abc\n123"

Consequently, this feature allows multiple HEREDOCs:

list = [~~ONE, ~~TWO, ~~THREE]
this is the first string
~~
this is another one
~~
this is the third. cool, isn't?
~~

list[0] % => "this is the first string\n"
list[1] % => "this is another one\n"
list[2] % => "this is the third. cool, isn't?\n"

Invoking Erlang Methods

Invoking Erlang methods with elixir is quite trivial:

% Accessing the is_atom BIF from Erlang.
% This is the same as `is_atom(foo)` in Erlang.
Erlang.is_atom('foo)  % => true

% Accessing the function delete from module lists.
% This is the same as `lists:member(1, [1,2,3])` in Erlang.
Erlang.lists.member(1, [1,2,3]) % => true

As there is no conversion between most Erlang data types and Elixir ones, there is no performance hit in invoking Erlang methods. The only exception are strings that are binaries in Elixir and may need to be converted to char lists in some specific erlang modules. More details were outline in the BitString and String sections above.

Finally, notice that Erlang is just a proxy that is converted to erlang calls at compile time.

List and Bit string comprehensions

List comprehensions allow you to quickly build a list from another list:

[n*2 for n in [1,2,3,4]]  % => [2,4,6,8]

The comprehension is defined with the for keyword which accepts several expressions. Those expressions can be generators, as in x in [1,2,3,4], or filters:

% A comprehension with a generator and a filter
[n for n in [1,2,3,4,5,6], X rem 2 == 0]  % => [2,4,6]

% A comprehension with two generators
[x*y for x in [1,2], y in [2,3]]  % => [2,3,4,6]

There are two types of generators in Elixir/Erlang: list and bit string generator:

% A list generator:
[n*2 for n in [1,2,3,4]]  % => [2,4,6,8]

% A bit string generator:
[n*2 for <<n>> in <<1,2,3,4>>]  % => [2,4,6,8]

Bit string generators are quite useful when you need to organize bit string streams:

pixels = <<213,45,132,64,76,32,76,0,0,234,32,15>>
[{r,g,b} for <<r:8,g:8,b:8>> in pixels ]  % => [{213,45,132},{64,76,32},{76,0,0},{234,32,15}]

Remember, as strings are binaries and a binary is a special kind of bit string where the number of bit is a multiple of 8, we can also use strings on comprehensions. For instance, the example below removes all white space characters from a string:

<<c for <<c>> in " hello world ", c != $\s>> % => "helloworld"

Elixir does its best to hide the differences between list and bit string generators from you. However, there is a special case due to Erlang limitation that you need to explicitly tell Erlang that a list is being given as argument:

% This will fail because when Elixir sees that the left side
% of the in expression is a bit string, it expects the right side
% to be a bit string as well:
[n*2 for <<n>> in [<<1>>,<<2>>,<<3>>]  % => [2,4,6]

% You need to be explicit and use inlist:
[n*2 for <<n>> inlist [<<1>>,<<2>>,<<3>>]  % => [2,4,6]

% inbin is also available:
[n*2 for <<n>> inbin <<1,2,3>>]  % => [2,4,6]

You can read more about list and bit string comprehensions in Learn You Some Erlang.

Modules

As Erlang, Elixir code is mainly organized around modules. Here is a very simple example:

module Hello
  def world
    IO.puts "Hello World"
  end
end

Hello.world % => "Hello World"

Method Visibility

One important aspect of modules is the method visibility. Elixir provides two different visibilities: public and private. All methods are public by default, this means that a method can be called from anywhere, at any time:

module Example
  def public_method
    13
  end

  def calling_public_method
    public_method
  end

  def calling_public_method2
    self.public_method
  end
end

Example.public_method           % => 13
Example.calling_public_method   % => 13
Example.calling_public_method2  % => 13

Private methods are internal and therefore cannot be accessed in the self.method format:

module Example
  def calling_private_method
    private_method
  end

  def calling_private_method2
    self.private_method
  end

  private

  def private_method
    13
  end
end

% Won't work, the method is private.
Example.private_method

% It works because calling_private_method is calling private_method without self.
Example.calling_private_method   % => 13

% It won't work because calling_private_method is calling private_method with self.
Example.calling_private_method2

Local and remote calls

In Erlang, it is very important to make a difference between local calls and remote calls, as they affect how hot code swapping works. You can read this section from Learn You Some Erlang for more information.

In Elixir, every time you call a private method, it is doing a local call. This means that private methods are always called locally (i.e. in the same module) and can't be overridden in mixins. Consider the following example:

module A
  def foo
    1 + bar + baz
  end

  def bar
    1
  end

  private

  def baz
    1
  end
end

module B
  mixin A

  def bar
    2
  end

  def baz
    3
  end
end

A.foo % => 3
B.foo % => 4

Notice how B.foo returns 4 and not 6. This is because we can only override bar in mixins, we can't access private methods as they can only be called locally (i.e. from the same module).

Finally, notice that if a variable is defined with the same name as method, the variable is given higher preference:

module AnotherExample
  def value
    some_value = 11
    some_value
  end

  private

  def some_value
    13
  end
end

AnotherExample.value % => 11

If in the example above you wanted to call do a local call, you could have used some_value() to disambiguate.

Tail call optimization

In the "Variables and Pattern Matching" section above, we have showed a simple Fibonacci example using Pattern Matching in the method signature. However, that example was not properly optimized:

module Math
  def fibonacci(0)
    0
  end

  def fibonacci(1)
    1
  end

  def fibonacci(n)
    fibonacci(n - 1) + fibonacci(n - 2)
  end
end

As Erlang, Elixir does tail call optimization. We can rewrite the fibonacci method with a version that will use tail call optimization like below:

module OptimizedMath
  def fibonacci(n)
    fibonacci(n, 1, 0)
  end

  def fibonacci(0, _, result)
    result
  end

  def fibonacci(n, next, result)
    fibonacci(n - 1, next + result, next)
  end
end

OptimizedMath.fibonacci(0)   % => 0
OptimizedMath.fibonacci(1)   % => 1
OptimizedMath.fibonacci(3)   % => 2
OptimizedMath.fibonacci(10)  % => 55

The third fibonacci method in OptimizedMath is optimized because the last method calls itself. In order to understand the difference between both versions and how tail call optimization works, we recommend reading more about it on the Recursion chapter from Learn You Some Erlang.

Pattern matching in methods

As we mentioned earlier and saw in the examples above, pattern matching is also allowed in method signatures. If the given args does not match a given method, it will try the next one until it succeeds or none is found, raising an error. Below, is an example that checks if a list is the prefix of another, relying solely on pattern matching:

module Prefix
  % This won't match if the first element of each list is not equal
  def is?([i|prefix], [i|list])
    is?(prefix, list)
  end

  % If prefix is empty or gets empty, it matches
  def is?([], _list)
    true
  end

  % Anything else is false
  def is?(_prefix, _list)
    false
  end
end

prefix = "foo".to_char_list
full   = "foobar".to_char_list
Prefix.is?(prefix, full) % => true
Prefix.is?(full, prefix) % => false

The fact OrderedDicts are allowed in pattern matching and pattern matching is allowed in methods, makes it possible to use key-value arguments:

def do_something(value, 'special: true)
  % Do something special
end

def do_something(value, 'special: false)
  % Do something not that special
end

Default arguments in methods

Besides supporting pattern matching in methods, Elixir also supports default arguments. You can specify a default argument using the := operator. Example:

module Default
  def sum(a := 1, b := 2)
    a + b
  end
end

Default.sum        % => 3
Default.sum(2)     % => 4
Default.sum(2, 3)  % => 5

Default arguments work by implicitly defining methods that accepts less arguments. The code above generates exactly the same module as follow:

module Default
  def sum()
    sum(1, 2)
  end

  def sum(a)
    sum(a, 2)
  end

  def sum(a, b)
    a + b
  end
end

Documentation

Module binding, refinements and mixins

Elixir provides a way to bind modules to structures in order to provide method dispatching. A couple of features are built on top of this functionality and are going to be described next.

Binding

Binding is the ability to bind modules to data types. Method dispatching is done by simply invoking a method in the module bound to the structure. For example, when the method + is invoked in a string, it is simply invoking the method + defined in String::Behavior as below:

module String::Behavior
  def +(another)
    <<self|binary, another|binary>>
  end
end

All built-in data types (like integers, tuples, lists, etc) are already bound to a given module at runtime and cannot be changed. However, Elixir provides a data structure called blank slate that is not bound by default and therefore can be bound to any module:

module Car
  def engine
    IO.puts "VROOOM"
  end
end

% Create a blank slate and bind it to `Car` module:
car = Module.blank_slate#Car()
car.engine % => "VROOOM"

The Module.blank_slate expression above returns an empty data type that is then bound to the module Car using the operator #. Since the expression Module.blank_slate#Car() is too long, the form commonly used is:

car = #Car()
car.engine % => "VROOOM"

Notice the # operator has the same precedence as ., so the next expressions are equivalent and will all print "VROOOM":

 Module.blank_slate#Car.engine
 (Module.blank_slate#Car).engine
 Module.blank_slate#Car().engine

Internal variables

Elixir allows us to store information inside blank slates. This is done with internal variables. In the example below, we are going to store the color of a car by using the __bound__ callback and then read it:

module Car
  % Callback invoked whenever this module is bound to a structure.
  % All the argument passed to the bind operator are accessible here.
  def __bound__(color)
    % Set the internal variable color to the given color.
    @('color, color)
  end

  def color
    % Read the internal variable @color.
    % @internal_variables are always relative to self.
    @color
  end
end

car = #Car('green)
car.color % => 'green

Whenever a module is bound, the callback __bound__ in the module is invoked. All the values given on binding are accessible in the callback. Data can be added to the blank slate through internal variables (for example, @color above).

Besides blank slates, modules also have internal variables:

module Car
  set_ivar('color, 'red)

  def color
    @color
  end
end

Car.color % => 'red

In the example above, we are setting and reading the internal variable of the module Car. However, keep in mind that if we bind the module, @color will point to the bind object as seen above:

#Car().color % => nil

Mutability

Almost everything in Elixir is immutable. For example, every time we set the internal variable of a new structure, it returns a new structure instead of modifying the first one in place:

car = #Car('green)
new_car = car.set_ivar('color, 'red)

car.color     % => 'green
new_car.color % => 'red

The only exception to mutability are modules:

module Car
  set_ivar('color, 'red)

  def color
    @color
  end
end

Car.color % => 'red

In the example above, set_ivar is setting the value of the internal variable @color but it is not returning a new object, it is modifying the value of the module in place! However, notice that modules are only mutable during definition. After a module is defined, we can no longer modify it:

new_module = Car.set_ivar 'color, 'green
Car.color        % => 'red
new_module.color % =>'green

Notice in the example above Car's internal variable @color is still 'red even after calling set_ivar. Immutability is important to ensure we won't have race conditions or deadlocks on runtime. However, it is ok for modules to be mutable as long as it happens only during compilation time, adding great extensibility.

Best practices

Libraries must hide the blank slate binding as most as possible. For instance, the Car example above, requires the developer to manually bind a method, leading to coupling. Ideally, the Car should provide an API for that as below:

module Car
  def new(color)
    #Car::Behavior(color)
  end

  module Behavior
    def __bound__(color)
      @('color, color)
    end

    def color
      @color
    end
  end
end

Refinements (to be implemented)

Refinements is the ability to change an already defined module at compile time avoiding the need to always bind the module to the same data types:

:refine Integer::Instance with DurationExtensions
1.day_in_seconds % => 3600

Mixins

Mixins are the ability to mix one module into another module still in definition:

module SimpleMath
  def one
    1
  end

  def two
    2
  end
end

module AdvancedMath
  mixin SimpleMath

  def one_plus_two
    one + two
  end
end

AdvancedMath.one_plus_two % => 3

Different from refinements that only alter method lookup, mixins provide a copy mechanism that copy all the methods defined in the mixed in module into the target providing faster behavior at run-time.

Note that mixed in methods are available straight away, while methods defined in the module are just available after the module is defined:

module AdvancedMath
  mixin SimpleMath

  one % => 1
  two % => 2

  def one_plus_two
    one + two
  end

  % Fails because AdvancedMath is still
  % in definition.
  one_plus_two % => ERROR
end

Finally, notice that methods that are not available at compile time cannot be called locally. For instance, imagine you have module RequiresName that requires name to be implemented in the target:

module RequiresName
  def shout
    IO.puts "HELLO #{self.name}"
  end
end

module Person
  mixin RequiresName

  def name
    "John Doe"
  end
end

Person.shout % => "HELLO John Doe"

In the example above, RequiresName uses self.name instead of name. This is required because name is not known locally.

Temporary mixins

Sometimes it is also convenient to include a mixin only during the module definition. This is achieved with temporary mixins:

module SimpleMath
  def one
    1
  end

  def two
    2
  end
end

module AdvancedMath
  using SimpleMath

  one + two % => 3

  def one_plus_two
    one + two
  end
end

% Will fail because SimpleMath methods are
% available only during the module definition.
AdvancedMath.one_plus_two % => ERROR

Summary

  • Binding allows us to change dispatch on a given data type at runtime;
  • Refinements allows us to change dispatch on an existing module at compile time;
  • Mixins allows us to mix behavior from existing modules during module definition.

Documentation

Code and load paths

Loading code in Elixir happens by automatically loading modules inside the compilation directory. For instance, if you are building a library and have the compiled code inside the exbin/ directory, you can access any of the modules in it using:

bin/elixir -pa exbin/ -e "SomeCompiledModule.method"

You can find more documentation by typing "bin/elixir". You may also add and remove paths programatically

When scripting, it may be convenient to load another specific script file, you can do that using Code.load_file or Code.require_file in which the second assures the file is being loaded just once.

Documentation

Processes, Behavior and Callbacks

Elixir provides the same facilities to deal with processes as Erlang. Messages are sent using <- and the same receive/after syntax is available. You can learn more about it by checking the process.ex file in the examples folder: https://github.com/josevalim/elixir/tree/master/examples/process.ex

Besides, Elixir also imports behaviors from Erlang OTP. Currently, just GenServer is implemented and support for others will come as needed. Once again, you can learn more in the examples folder: https://github.com/josevalim/elixir/tree/master/examples/gen_server.ex

Documentation

Advanced Topics

Some advanced topics related to Elixir.

Variable scopes

As explained at the beginning of this README, Elixir allows the same variable to be assigned more than once. However, keep in mind that variables assignment inside functions do not change the original binding. For example:

a = 1
b = -> a = 2
b()
a % => 1

As everything is immutable, when the function assigns a new variable, it creates a new binding with the new variable value and the original binding is never modified. This is important to avoid side-effects when passing functions to different processes.

Also, Elixir has much more flexible rules when it comes to variables inside control-flow expressions. For instance, the following works:

x = 1

if true
  x = 2
end

x % => 2

The same is also true for receive/after and case/match expressions. The only exception comes to try/catch scenarios, where a variable defined inside such blocks is never accessible from the outside. For example:

x = 1

try
  x = 2
catch _:_
  % Do nothing
end

x % => 1

Guards

Elixir has basic support for guards. They can be used on method declaration, receive/match clauses, case/match clauses and catch clauses. In all cases, they are declared using the keyword when. For instance, you could implement a method that returns the absolute value of a number as follow:

def abs(x) when x < 0
  - x
end

def abs(x)
  x
end

In a receive/case match clause, we would do instead:

case y
match x when x < 0 then - x
match x then x
end

Finally, in catch expressions it works as follow:

try
  throw y
catch 'throw:x when x < 0
  - x
catch 'throw:x
  x
end

Guards only supports arithmetic operators on numbers, comparison operators and the following boolean operators: or, orelse, and, andalso and not.

Dynamic Dispatch and Metaprogramming

Elixir allows you to dynamically dispatch methods using send:

[1,2,3].send 'head   % => 1
{}.send 'empty?      % => true

Elixir also allows you to dynamically define methods. For example, below we can define attribute readers for both "title" and "author" attributes dynamically:

module Person
  def __bound__(name, age)
    @('name: name, 'age: age)
  end

  ["name", "age"].each do (method)
    module_eval __FILE__, __LINE__ + 1, ~~METHOD
  def #{method}
    @#{method}
  end
~~
  end
end

person = #Person("John Doe", 24)
person.name % => "John Doe"

The real benefit is when you encapsulate it inside a method. For example, the definition above is inside Elixir, so you can actually call:

module Person
  attr_reader ['name, 'age]

  def initialize(name, age)
    @('name: name, 'age: age)
  end
end

Documentation

Performance

The focus in Elixir so far has not been in performance, but there are a few things you can do right now.

Compilation to Native Code

Elixir can compile to native code using the Hipe compiler. All you need to do is to export the following before running your code:

export ERL_COMPILER_OPTIONS=native

Records

Elixir allows you to import records from Erlang code. Here is an example that imports the file_info record available in the kernel module:

module FileInfo
  mixin Record
  record 'file_info, 'from_lib: "kernel/include/file.hrl"
end

% Manually access the Erlang file:read_file_info method
% passing the current file as a char list.
{ 'ok, info } = Erlang.file.read_file_info(__FILE__.to_char_list)

% Create a new FileInfo object based on the tuple returned above
record = #FileInfo(info)

% Profit by accessing the record info
record.access % => 'read_write

Documentation

License

Copyright (c) 2011 José Valim

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

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