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 syntax and object orientation. Elixir provides a very simple Object Model based on prototype languages with most of its syntax based on Ruby.

Usage

Elixir is not ready for use yet, though this milestone will be achieved soon. If you want to contribute to Elixir, you can do that by cloning the repository and running make test.

The tests are organized in two directories: test/erlang and test/elixir. The first are written in Erlang and the second in Elixir. Much of Elixir's standard library is written in Elixir and tested in Elixir. This makes fairly easy to improve the language.

Roadmap

• Add exceptions
• Add load paths
• Add interactive elixir (iex)
• Implement missing types and improve STDLIB
• Add generators/list comprehensions
• Add metaprogramming
• Add partial application, pipeline f1 + f2, and 1#add and Integer##add
• Add _.foo
• Support guards
• Add JIT on module compilation
• Extending builtin types (like inheriting from Integer)
• Add method cache table
• Allow object definitions to be reopened or to copy from another object
• Improve constant lookup

Learning Elixir

This is a basic introduction into Elixir basic objects and object model. 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.

Some notation

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 * 100 - 490     % => 10
(50 * 100) - 490   % => 10
-(50 * 100) - 490  % => -990

As in Ruby, everything is an object, so we can call methods on numbers:

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

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

1.+(2)  % => 3

Documentation:

• https://github.com/josevalim/elixir/tree/master/lib/integer.ex
• https://github.com/josevalim/elixir/tree/master/lib/float.ex
• https://github.com/josevalim/elixir/tree/master/lib/numeric.ex

To be implemented

Currently, there is no support to enter numbers in bases other than base 10. This is the current API in Erlang:

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

Another feature that will likely be included is the ability to include "_" in numbers as in Ruby. This improves the readability when working with large numbers:

1_052_672

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:

• https://github.com/josevalim/elixir/tree/master/lib/atom.ex

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

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
{ }

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:

• https://github.com/josevalim/elixir/tree/master/lib/tuple.ex

To be implemented

Implement negative indexes:

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

Setting an element should be done through the set method:

{ 'a, 'b, 'c }.set(0, 'd) % => { 'd, 'b, 'c }

And finally decide how out of bound indexes will behave.

Lists

Lists are the main object 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].head         % => 1
[1, 2, 3].tail         % => [2,3]
[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.

Nil value

A nil value in Elixir is represented by an empty list. In if expressions (that we briefly saw above), everything evaluates to true, except empty lists [] and false. Both examples below will return 1 as result:

if 'any_symbol
  1
else
  2
end

if []
  2
else
  1
end

Documentation:

• https://github.com/josevalim/elixir/tree/master/lib/list.ex

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:

• https://github.com/josevalim/elixir/tree/master/lib/ordered_dict.ex

Binaries

Elixir has a similar syntax to Erlang for handling binaries:

% A binary with three elements
<<1, 17, 42>>

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

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

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

By default, the binary 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 binaries:

<<1+2>>

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

Documentation

• https://github.com/josevalim/elixir/tree/master/lib/binary.ex
• http://www.erlang.org/doc/programming_examples/bit_syntax.html

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.

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

% A string converted to its underlying binary:
"hello".to_bin  % => <<[104, 101, 108, 108, 111]>>

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

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

This difference is important because strings are the only object that needs conversion between Elixir and Erlang. Erlang methods that expect Strings actually expect a binary or a list of characters, so you need to convert any Elixir string before passing them to Erlang.

On the other hand, if you receive a String from an Erlang method you are actually receiving a binary or a list of characters, so you may want to typecast to Elixir's string. Summing up, here are the conversions you need to know:

% Converting a string_from_erlang to Elixir's String
String.new string_from_erlang

% Where string_from_erlang is either a binary:
<<[104, 101, 108, 108, 111]>>

% Or a char_list:
[104, 101, 108, 108, 111]

% Converting a string_from_elixir to Erlang
"string_from_elixir".to_bin
"string_from_elixir".to_char_list

Finally, strings also support interpolation:

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

Documentation

• https://github.com/josevalim/elixir/tree/master/lib/string.ex

Functions

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

my_function = do
  1 + 2
end

my_function() % => 3

another_function = ->
  1 * 2
end

another_function() % => 2

Some functions expect arguments:

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

my_function(1, 2) % => 3

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

another_function(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(1, 2) % => 3

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

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

% This works as expected:
my_function = -> 1 + 2
my_function() % => 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 parenthesis is part of the argument list or the function expressions. This is why you either need to remove parenthesis (as in the first example) or add empty parenthesis (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 parenthesis:

[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 parenthesis, improves readability:

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

One final note is, while parenthesis are optional for method invocations, function invocations always require parenthesis:

my_function = -> (x, y) x + y

% This won't work and will raise undefined method my_function
my_function 1, 2

% This works
my_function(1, 2)

Documentation

• https://github.com/josevalim/elixir/tree/master/lib/function.ex

Variables and Pattern Matching

Elixir inherits single assignment variables and pattern matching from Erlang. This means there isn't an assignment operator, but rather a match operator. A variable can be assigned just once:

x = 1
x = 2  % => Raises a bad match error, because x is already equals to 1

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

However, 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

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

We will discuss modules and methods with more details 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 objects types:

number < atom < reference < fun < port < pid < tuple < any other object < 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
+		any object
-		any object
*		any object
/	returns a float	any object
div	returns an integer	any object
rem	returns an integer	any object

Except by the two unary operators, all other operators accept any object as parameter. This is because those operators are implemented as methods and their implementation are defined by the object which is receiving the method. 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.

Logical operators and control-flow

Elixir provides three operators that accept any object as argument. We will see later that some operators (inherited from Erlang) accept strictly boolean values.

Operator	Description
&&	and
||	or
!	not

Remember that any object, except false and [] (empty list), evaluates to true:

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

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

[] && true   % => []
true && []   % => []
1 && 2       % => 2

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

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

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

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

Precedence

Operator precedence in falling priority:

Operator	Associativity
+ - ! not	Non associative (unary operators)
/ * div rem	Left
== != < <= > >= =:= =!=	Left
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 [] (empty list) 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 invoking 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, providing a more compact syntax:

case {4,5,6}
match {3,2,x} then x * 2
match {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

To be written/implemented.

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

• https://github.com/josevalim/elixir/tree/master/lib/string.ex
• https://github.com/josevalim/elixir/tree/master/lib/atom.ex
• https://github.com/josevalim/elixir/tree/master/lib/regexp.ex

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

Notice that Erlang is not a real object in Elixir, but just a proxy that is converted to erlang calls at parse time.

The Object Model

This section will discuss Elixir's Object Model. Its main aspects are:

• Dynamic Dispatch - when a method is invoked on an object, the object itself determines which code gets executed
• Mixins - an object does not contain methods, all methods are packed into modules that are mixed into objects
• Encapsulation - methods can either be public, protected or private
• Open recursion - Elixir's has a special variable called self that allows a method body to invoke another method body of the same object, passing through the ancestors chain
• Reflections - Elixir is able to observe and modify an object structure at runtime

Why Objects?

Elixir's Object Model focuses on method dispatching. Imagine you have a Person structure with fields name and age generated by an ORM. Retrieving this Person structure, changing its name and saving it would be as follow in Erlang:

Person = person:find(john),
ModifiedPerson = person:set(name, john_doe, Person)
true = person:save(ModifiedPerson)

In Elixir, this would be written as:

person = Person.find('john)
modified_person = person.set('name, 'john_doe)
true = person.save

In Erlang, every time we want to do something with the Person structure, we need to explicitly call the module person and pass the Person structure as parameter.

Elixir is more concise due to method dispatching. Once you create a person object, you can invoke methods on it directly and there isn't a need to always pass the own list object as argument. The need for dynamic method dispatching is one of the main reasons for the existence of Elixir and its Object Model.

How does it work?

In Elixir, everything is an object. Objects carry properties, they don't carry methods. We can define a new object as follow:

% Define an object Person
object Person
end

% Creates an instance of this object
Person.new

In Elixir, modules carry behavior, i.e. methods. All modules are objects, but not all objects are modules. We can define a new module Speak with a method called say that receives a message and prints it out as follow:

module Speak
  def say(message)
    IO.puts message
  end
end

The convenience of objects comes when we add modules to objects:

person = Person.new
person.say "Hi"          % => Raises no method error

another_person = Person.new.mixin(Speak)
another_person.say "Hi"  % => "Hi"

The mixin method adds the given module to the current object mixins chain. Therefore, every time we invoke a method in this object, it will search if the method exists in one of its mixins. We can retrieve all mixins of an object with the method __mixins__:

another_person.__mixins__  % => ['Speak, 'Object::Methods]

The Speak module is the one we added and Object::Methods is a module included by default in all objects and is where the __mixins__ method is implemented.

If every time we create a new Person instance, we need to explicitly mix in a module, Elixir wouldn't be much useful. For this reason, we define in the object Person that all of its children has a Speak module by default. This can be done with the proto method:

object Person
  proto Speak
end

Person.new.say "Hi"  % => "Hi"

Person.__protos__      % => ['Speak, 'Object::Methods]
Person.new.__mixins__  % => ['Speak, 'Object::Methods]

A proto is how an object specifies how its children is going to behave. In other words, a proto added to the parent, becomes a mixin to the child.

Since everything is an object, the mixin method we discussed earlier is also available to the Person object:

module NewBorns
  def create_and_cry
    instance = self.new
    instance.say "whaaaaaaa"
    instance
  end
end

object Person
  mixin NewBorns
  proto Speak
end

person = Person.create_and_cry  % => "whaaaaaaa"

Person.__mixins__    % => ['NewBorns, 'Object::Methods]
Person.__protos__    % => ['Speak,    'Object::Methods]

This wraps the core of Elixir object system. Remember: everything is an object, methods are defined in modules and modules can be either mixed into objects (mixin), changing their current behavior, or added as prototype (proto), which will define the behavior of all children of that object. Finally, all modules defined as proto in the parent, becomes a mixin to the child.

On top of that, Elixir provides a better way to organize our code. Let's rewrite our Person object:

object Person
  module Mixin
    def create_and_cry
      instance = self.new
      instance.say "whaaaaaaa"
      instance
    end
  end

  module Proto
    def say(message)
      IO.puts message
    end
  end

  mixin Person::Mixin
  proto Person::Proto
end

Instead of defining modules without a namespace, we can define them inside the Person object, avoiding polluting the main namespace and reducing the chance of conflicts.

Finally, since the pattern above is very common, Elixir provides three conveniences to make our code more expressive:

• If a module Mixin is defined inside an object, it is automatically added as mixin;
• If a module Proto is defined inside an object, it is automatically added as proto;
• You can also completely skip the proto module and define methods as if you were defining methods inside the object. This is a just a syntax convenience. Internally, Elixir will still create a Proto module and automatically add it as proto to your object.

With these conveniences in mind, let's rewrite our Person object once again:

object Person
  module Mixin
    def create_and_cry
      instance = self.new
      instance.say "whaaaaaaa"
      instance
    end
  end

  def say(message)
    IO.puts message
  end
end

This is much better! Now let's prove that everything is the same as before:

Person.__mixins__    % => ['Person::Mixin, 'Object::Methods]
Person.__protos__    % => ['Person::Proto, 'Object::Methods]

Instance variables

When creating an object, we sometimes want to define properties specific to that object. For example, a Person may have name and age as properties. This can be done by defining a constructor. A constructor is a method that receives all arguments given to new and should return a dict. The key-values given by the dict can be retrieved using instance variables:

object Person
  def constructor(name, age)
    { 'name: name, 'age: age }
  end

  def name
    @name
  end

  def age
    @age
  end
end

person = Person.new('john, 24)
person.name % => 'john
person.age  % => 24

Instance variables can be changed using the set_ivar method:

object Person
  def constructor(name, age)
    { 'name: name, 'age: age }
  end

  def name
    @name
  end

  def age
    @age
  end

  def name(value)
    self.set_ivar('name, value)
  end
end

person = Person.name('john, 24)
another_person = person.name('john_doe)

person.name % => 'john
person.age  % => 24

another_person.name % => 'johh_doe
another_person.age  % => 24

Notice in the example above that set_ivar returns a new object. This is expected because as Erlang structures are immutable, all objects in Elixir are also immutable. Above we can see that the initial person object has not changed at all.

Advanced: The Object Graph

One final note about the object model is how instantiation works. When you create an instance from Person, it annotates that the parent for that instance is the Person object. Let's take a look at it:

person = Person.new
person.__parent__   % => 'Person

The Person object is a direct child from Object:

Person.__parent__  % => `Object`

While all modules are children from Module which is a child from Object. The object Object, has no parent:

Person::Mixin.__parent__ % => 'Module
Person::Proto.__parent__ % => 'Module
Module.__parent__        % => 'Object
Object.__parent__        % => []

The object Object defines Object::Methods as proto, this is why all objects have this method as their mixin. It doesn't matter if you are a child, a grandchild or a grand-grandchild from Object, this mixin will be available to you. This happens because, in order to calculate all mixins for a given object, Elixir traverses the whole ancestors chain getting all modules defined as proto for all parents.

The Object Graph for all these objects can be seen below:

---------------    Parent   -----------------
|    Object   | <---------- |     Module    | <---
---------------             -----------------    |
      ^                            ^             |
      |  Parent                    |  Parent     |
      |                            |             |
---------------    mixin    -----------------    |
|   Person    | <---------- | Person::Mixin |    | Parent
---------------  _          -----------------    |
      ^         |\__ proto                       |
      |  Parent     \___                         |
      |                 \__                      |
---------------            \-----------------    |
| Person.new  | <---------- | Person::Proto | ---|
---------------    mixin    -----------------

Once again, remember:

• Everything is an object;
• Methods are defined in modules. All modules are all objects, but not all objects are modules. Besides, modules cannot have instances. Person::Mixin and Person::Proto defined above are modules;
• Modules can be either mixed into objects (mixin), changing their current behavior...
• Or added as prototype (proto), which will define the behavior of all children/instances from that object;
• Finally, all modules defined as proto in the parent, becomes a mixin to the child.

Documentation

• https://github.com/josevalim/elixir/tree/master/lib/object.ex

Modules

In the Object Model section, we have discussed modules and methods. Modules are very close to Erlang modules and it is important to keep that in mind if you are coming from a language like Ruby.

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.

Elixir keeps the same semantics as Erlang and makes a difference between local and remove calls. Let's take a look at some examples:

module Example
  def remote_call
    self.__parent__
  end

  def local_call
    remote_call
  end

  def invalid_local_call
    __parent__
  end
end

% Remember the parent for a module is 'Module
Example.__parent__   % => 'Module
Example.remote_call  % => 'Module
Example.local_call   % => 'Module

% This won't work, in fact, it wouldn't even compile
Example.invalid_local_call

In Elixir, every time you invoke a method without an explicit receiver (like self), you are making a local call. A local call searches for a method in the current module and, if no method exists, a compilation error is raised.

This means that, even though all objects have a __parent__ method, if you call __parent__ without an explicit receiver inside a method, it will try to find a local method __parent__ defined inside the current module. In order to really dispatch a method that goes through all mixins for that object, you need to use self as explicit receiver.

Note that this rule applies only expressions inside methods. Take this for example:

module MoreExamples
  mixin Something
end

Even though mixin is a method that comes from Object, we can invoke it without an explicit self. This doesn't cause conflicts because you can only invoke a method in a module after the module is completely defined. Therefore, this doesn't work:

module MoreExamples
  def foo
    10
  end

  % Raises no method error
  foo

  % Raises undefined constant MoreExamples
  MoreExamples.foo
end

Finally, as local calls have the same syntax as variables. If a variable is defined with the same name as method, the variable is given higher preference:

module AnotherExample
  def some_value
    13
  end

  def value
    some_value = 11
    some_value
  end
end

AnotherExample.value  % => 11

Method Visibility

Now that we know the difference between local and remote calls we can take a better look at method visibility. Elixir provides three different visibilities: public, protected 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

A public method can be called from another module, as long as it is a remote call:

module Invoker
  mixin Example

  % This won't work (it won't even compile) because, as we saw
  % previously, it will attempt to call public_method locally.
  % def calling_public_method
  %   public_method
  % end

  def calling_public_method
    self.public_method
  end
end

Invoker.public_method          % => 13
Invoker.calling_public_method  % => 13

A protected method can only be called if the current scope includes the mixin the module belongs to. Some examples:

module Example
  def calling_protected_method
    public_method
  end

  def calling_protected_method2
    self.public_method
  end

  protected

  def protected_method
    13
  end
end

% Here the current scope (self) is Object which doesn't have Example as mixin,
% so invoking the protected method raises an error.
Example.protected_method

% calling_protected_method calls protected_method using a local call.
% A local call always work, as it means they are in the same module.
Example.calling_protected_method  % => 13

% The following also works because calling_protected_method2 and
% protected_method are defined in the same module.
Example.calling_protected_method2 % => 13

module Invoker
  mixin Example

  def calling_protected_method
    % Here the current scope (self) is Invoker, that has the mixin Example
    % So calling Invoker.calling_protected_method at any point will work
    self.protected_method
  end
end

Invoker.calling_protected_method  % => 13

Finally, private methods are the ones accessible just through a local call. This means a module cannot access private methods from other modules even after adding them as mixin or as proto.

module Example
  def calling_private_method
    private_method
  end

  def calling_private_method2
    self.private_method
  end

  protected

  def private_method
    13
  end
end

% Won't work, it is not a local call.
Example.private_method

% It works because calling_private_method is doing a local call.
Example.calling_private_method   % => 13

% It won't work because calling_private_method2 is not doing a local call.
Example.calling_private_method2

module Invoker
  mixin Example

  def calling_private_method
    self.private_method
  end
end

% It won't work because private_method is only accessible from
% local calls from the same module it is defined.
Invoker.calling_private_method

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 (though it only applies to local calls). 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 function is optimized because the last method it calls is 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.

Retrieving a method as a function

Before proceeding on how to retrieve a method as a function, it is important to notice that, as in Erlang, Elixir's methods are identified by its name and arity. Therefore, the OptimizedMath module above has only two methods: a fibonacci with arity 1 and fibonnaci with arity 3. If two methods are defined with same name and arity, they become different clauses for the same method and pattern matching is used in order to specify which method to call. That said, the Math module has only one fibonnaci method with arity equals to 1 and 3 clauses.

Remaining of this section still needs to be implemented and written.

Documentation

• https://github.com/josevalim/elixir/tree/master/lib/module.ex

Behavior and Callbacks

To be written.