Elixir Cheat Sheet

elixir -v
1.3.1

Elixir is a dynamic functional compiled language that runs over an Erlang Virtual Machine called BEAM.

Erlang and its BEAM is well known for running low-lattency, distributed and fault-tolerant applications.

Elixir was designed to take all that advantages in a modern coding language.

Elixir data types are immutable.

In Elixir a function is usually described with its arity (number of arguments), such as: is_boolean/1.

File Types

• .exs => Elixir script file
• .ex => Elixir regular file
• .beam => Compiled Elixir file

Compile and Run Elixir code

• elixir <script_file>.exs => run a script file
• elixirc <file>.ex => compile a file to Elixir.<File>.beam

Elixir Conventions

• foo function return tuple => the result of a foo function is usually {:ok, result} or {:error, reason}
• foo! function may raise an error => the result of a foo! is not wrapped in a tuple and it may raises an exception
• Exceptions/Errors are not used for controlling flow
• Elixir uses fail fast idea and the supervision trees to control process health and possible restart processes.

Comments

• # => single line comment

There are no multi-line comment

Code Documentation

• @moduledoc => module documentation
• @doc => function documentation
• @spec => function arguments/return specification
• @typedoc => type documentation
• @type => type definition
• @typep => private type definition

defmodule Math do
  @moduledoc """
  Provides math-related functions.

  ## Examples

      iex> Math.sum(1, 2)
      3
  """

  @spec sum(number, number) :: number
  @doc """
  Calculates the sum of two numbers.
  """
  def sum(a, b), do: a + b
end

h Math
h Math.sum

Elixir Special Unbound Variable

• _ => unbound variable

Elixir/Erlang Virtual Machine inspection

• :observer.start => start a gui tool for inspection
• :erlang.memory => inspect memory
• :c.memory => inspect memory

:c.memory
# [
#   total: 19262624,
#   processes: 4932168,
#   processes_used: 4931184,
#   system: 14330456,
#   atom: 256337,
#   atom_used: 235038,
#   binary: 43592,
#   code: 5691514,
#   ets: 358016
# ]

Interactive Elixir

• iex => open Interactive Elixir
• iex <file> => open Interactive Elixir loading a file
• <Ctrl>c + a => close iex
• i <object> => information about an object
• h <function/arity> => help for a function
• h <operator/arity> => help for a operator
• s <function/arity> => specification for a function
• s <operator/arity> => specification for a operator
• t <function/arity> => type for a function
• c <file> => load and compile a .ex file

Basic Types

Integer

• 1 => integer
• 1_000 => integers can use _ to make it easy to read
• 0x1F => integer
• 0b1010 => binary integer notation 10
• 0o777 => octadecimal integer notation 511
• 0x1F => hexadecimal integer notation 31

Float

• -1.0 => float
• 5.7e-2 => float exponent notation 0.057

Atom

• :atom => atom / symbol

• true => boolean (atom)

BitString

• <<97::size(2)>> => bit string
• <<97,98>> => binary
• "elixir" => string

Tuple

• {1, 2, 3} => tuple

List

• [1, 2, 3] => list

• 'elixir' => char list

• [a: 5, b: 3] => keyword list short notation

• [{:a, 5}, {:b, 3}] => keyword list long notation

Map

• %{name: "Mary", age: 29} => map short notation (keys must be atoms)
• %{:name => "Mary", :age => 29} => map long notation

PID

• self() #=> #PID<0.80.0> => current Process id

Function

• fn -> :hello end => anonymous function

Reference

• make_ref() #=> #Reference<0.0.8.133> => create a new reference

Port

• hd Port.list() #=> #Port<0.0> => get first port

Type Testing

• is_nil/1
• is_integer 1
• is_float 4.6
• is_number 7.8
• is_atom :foo
• is_boolean false
• is_bitstring <<97:2>>
• is_binary <<97, 98>>
• is_list/1
• is_tuple/1
• is_map/1
• is_pid self()
• is_function(fn a, b -> a + b end) => function
• is_function(fn a, b -> a + b end, 2) => function with arity
• is_port hd Port.list()
• is_reference make_ref()
• Range.range?(1..3)

Converting Types

• to_char_list("hełło") => convert a string to char list
• to_string('hełło') => convert a char list to string
• Map.to_list(%{:a => 1, 2 => :b}) => convert a map to list of tuples or keyword list

Number Operators

• 10 / 2 => 5.0 => division always return a float
• div(10, 2) => 5 => integer division
• rem 10, 3 => 1 => integer remain of a division
• round(3.58) => 4 => float round
• trunc(3.58) => 3 => float trunc

Boolean Operators

Falsy in Elixir is false and nil, otherwise will be truthy.

• == => equals
• != => different
• === => strict equal (integer with float)
• !== => strict different (integer with float)
• < => less
• <= => less or equal
• > => greater
• >= => greater or equal
• && => truthy and
• || => truthy or
• ! => truthy not
• and => boolean and
• or => boolean or
• not => boolean not

It's possible to compare different data types and that's the sorting order: number < atom < reference < functions < port < pid < tuple < list < bit string.

Pipe Operator

• |> => pipe operator

The return of a function will be passed as the first argument to the following.

1..100 |>
  Stream.map(&(&1 * 3)) |>
  Stream.filter(&(rem(&1, 2) != 0)) |>
  Enum.sum
#=> 7500

Pattern Matching

In Elixir = sign is not just an assign operator, but a Match Operator.

This means that you assign variables from right side to the left based on patterns defined by the left one. If a pattern does not match a MatchError is raised.

This powerful tool is also used to decompose complex objects like tuples, lists, etc into smaller ones:

x = 1 #=> assign 1 to x
2 = x #=> ** (MatchError)
1 = x #=> match and does not assign anything

<<0, 1, x>> = <<0, 1, 2, 3>> #=> ** (MatchError)
<<0, 1, x::binary>> = <<0, 1, 2, 3>>
<<0, 1>> <> <<x::binary>> = <<0, 1, 2, 3>>
<<0, 1>> <> <<x, y>> = <<0, 1, 2, 3>>
<<0, 1>> <> <<x>> <> <<y>> = <<0, 1, 2, 3>>

"world" <> x = "hello" #=> ** (MatchError)
"he" <> x = "hello"

{x, y, z} = {1, 2} #=> ** (MatchError)
{} = {1, 2} #=> ** (MatchError)
{:a, :b} = {:b, :a} #=> ** (MatchError)
{x, y} = {1, 2}

first..last = 1..5

[x, 4] = [:a, 5] #=> ** (MatchError)
[] = [:a, 5] #=> ** (MatchError)
[:a, :b] = [:b, :a] #=> ** (MatchError)
[x, 4] = [:a, 4]

[x | y] = [] #=> ** (MatchError)
[x | y] = [1]
[x | y] = [1, 2, 3]

[a: x] = [b: 9] #=> ** (MatchError)
[a: x] = [a: 5]
[{:a, x}] = [a: 5]

%{a: x} = %{b: 5} #=> ** (MatchError)
%{} = %{a: 5} # empty map matches any map
%{a: x, b: 5} = %{b: 5, a: 7, c: 9}

defmodule User do
  defstruct name: "John", age: 27
end
john = %User{age: 29}
%User{name: name} = john
name #=> "John"

So in other words:

• non variables on the left side will be used to restrict a pattern to match
• variables using the pin operator on the left side will have its value to be used to restrict a pattern to match
• variables on the left side will be assigned with right side values

So variables and non variables behave differently with the match operator.

In order to assert an empty map you have to use a guard instead of pattern match, just like:

(
  fn m when map_size(m) == 0 ->
    "empty map"
  end
).(%{}) #=> "empty map"

Pin Operator

The Pin Operator ^ is used to treat variables the same way non variables with the match operator. In other words, the Pin Operator will evaluate the variable and use its value to restrict a pattern, preserving its original value.

x = 1 #=> assign 1 to x
^x = 1 #=> match x value with right side 1
^x = 2 #=> ** (MatchError)

Match Operator Limitation

You cannot make function calls on the left side of a match.

• length([1, [2], 3]) = 3 #=> ** (CompileError) illegal pattern

Custom Operators

You can customize an Elixir Operator like the following example:

1 + 2 #=> 3
defmodule WrongMath do
  def a + b do
    {a, b}
  end
end
import WrongMath
import Kernel, except: [+: 2]
1 + 2 #=> {1, 2}

Sigils

Available delimiters for Sigil: /, |, ", ', (, [, {, <.

• ~r => regular expression (modifiers: i)
• ~r/hello/i => i modifies to case insensitive
• ~w => list of string words (modifiers: )
• ~w[foo bar]c => c modifies to list of char lists
• ~w[foo bar]a => a modifies to list of atoms

~w(one two three) #=> ["one", "two", "three"]
~w(one two three)c #=> ['one', 'two', 'three']
~w(one two three)a #=> [:one, :two, :three]

Bit Strings

• <<97::4>> => short notation with 4 bits
• <<97::size(4)>> => long notation with 4 bits
• byte_size(<<5::4>>) => bit string byte size

Performance for Bit Strings:

cheap functions:

• byte_size(<<97::4>>)

expensive functions:

Binaries

Binaries are 8 bits multiple Bit Strings. Binaries are 8 bits by default.

• <<97>> => short notation with 8 bits
• <<97::size(8)>> => long notation with 8 bits
• <> => concatenate binaries/strings

Performance for Binaries:

cheap functions:

• byte_size(<<97>>)

expensive functions:

Strings

String is a Binary of code points where all elements are valid characters. Strings are surrounded by double-quotes and are encoded in UTF-8 by default.

• "hello" => string
• <<97, 98>> => string "ab"
• <<?a, ?b>> => string "ab" ?x => code points (ASCII code) for letter x
• "hello #{:world}" => string interpolation
• "\n" => new line
• String.length("hello") #=> 5 => get the length of a string
• String.upcase("hello") #=> "HELLO" => upcase a string
• """ => multi-line string begin/end

Performance for Strings:

cheap functions:

• byte_size("hello")

expensive functions:

• String.length("Hello")

Tuples

Tuple is a list that is stored contiguously in memory.

• {:ok, "hello"}
• tuple_size({:ok, "hello"}) => tuple size
• elem({:ok, "hello"}, 0) => get tuple element by index
• put_elem({:ok, "hello"}, 1, "world")

Performance for Tuples:

cheap functions:

• tuple_size({:ok, "hello"})
• elem({:ok, "hello"}, 1)

expensive functions:

• put_elem({:ok, "hello"}, 1, "world")

Lists

Lists implements Enumerables protocol.

List is a linked list structure where each element points to the next one in memory. When subtraction just the first ocurrence will be removed.

• [:ok, "hello"]
• [97 | [1, 6, 9]] => prepend
• [1, 5] ++ [3, 9] # [1, 5, 3, 9] => concatenation
• [1, 5] -- [9, 5] # [1] => subtraction first occurrences
• hd([1, 5, 7]) => head
• tl([1, 5, 7]) => tail
• :foo in [:foo, :bar] #=> true => in operator

Performance for Lists:

cheap functions:

• [97 | [1, 6, 9]] => prepend
• hd([1, 5, 7]) => head
• tl([1, 5, 7]) => tail

expensive functions:

• [1, 5] ++ [3, 9] # [1, 5, 3, 9] => concatenation
• [1, 5] -- [9, 5] # [1] => subtraction first occurrences
• length([1, 4])
• :foo in [:foo, :bar] #=> true => in operator

Char List

A Char List is a List of code points where all elements are valid characters. Char Lists are surrounded by single-quote and are usually used as arguments to some old Erlang code.

• 'ab' => char list
• [97, 98] => 'ab'
• [?a, ?b] => 'ab'
• ?x => code points (ASCII code) for letter x
• 'hello' ++ 'world' # 'helloworld' => concatenation
• 'hello' -- 'world' # 'hel' => subtraction first occurrences
• ?l in 'hello' #=> true => in operator

Performance for Char Lists:

cheap functions:

• [?H | 'ello'] => prepend

expensive functions:

• 'hello' ++ 'world' # 'helloworld' => concatenation
• 'hello' -- 'world' # 'hel' => subtraction first occurrences
• length('Hello')
• ?l in 'hello' #=> true => in operator

Keyword Lists

Keyword list is a list of tuples where first elements are atoms. When fetching by key the first match will return. If a keyword list is the last argument of a function the square brackets [ are optional.

• [a: 5, b: 3] => keyword list short notation
• [{:a, 5}, {:b, 3}] => keyword list long notation
• [{:a, 6} | [b: 7]] # [a: 6, b: 7] => prepend
• [a: 5] ++ [a: 7] # [a: 5, a: 7] => concatenation
• [a: 5, b: 7] -- [a: 5] # [b: 7] => subtraction first ocurrences
• ([a: 5, a: 7])[:a] # 5 => fetch by key
• length(a: 5, b: 7) => optional squared brackets [

Performance for Keyword Lists:

cheap functions:

• [{:a, 6} | [b: 7]] # [a: 6, b: 7] => prepend

expensive functions:

• [a: 5] ++ [a: 7] # [a: 5, a: 7] => concatenation
• [a: 5, b: 7] -- [a: 5] # [b: 7] => subtraction first ocurrences
• ([a: 5, a: 7])[:a] # 5 => fetch by key
• length(a: 5, b: 7) => optional squared brackets [

Maps

Maps implements Enumerables protocol.

Map holds a key value structure.

• %{name: "Mary", age: 29} => map short notation (keys must be atoms)
• %{:name => "Mary", :age => 29} => map long notation
• %{name: "Mary", age: 29}[:name] => fetch :name hash notation
• %{name: "Mary", age: 29}[:born] => returns nil when do not find in the hash notation
• %{name: "Mary", age: 29}.name => fetch :name short notation
• %{name: "Mary", age: 29}.born # ** (KeyError) => blows an error when key does not exists
• %{%{name: "Mary", age: 29} | age: 31} => update value for existing key
• %{%{name: "Mary", age: 29} | born: 1990} # ** (KeyError) => blows an error when updating non existing key
• map_size(%{name: "Mary"}) #=> 1 => map size

Performance for Maps:

cheap functions:

• %{name: "Mary", age: 29}[:name] => fetch :name
• %{name: "Mary", age: 29}.name => fetch :name short notation
• %{%{name: "Mary", age: 29} | age: 31} => update value for existing key
• map_size(%{name: "Mary"}) #=> 1 => map size

expensive functions:

Structs

Structs are built in top of Map.

• defstruct => define a struct

defmodule User do
  defstruct name: "John", age: 27
end
john = %User{} #=> %User{age: 27, name: "John"}
mary = %User{name: "Mary", age: 25} #=> %User{age: 25, name: "Mary"}
meg = %{john | name: "Meg"} #=> %User{age: 27, name: "Meg"}
bill = Map.merge(john, %User{name: "Bill", age: 23})

john.name #=> John
john[:name] #=> ** (UndefinedFunctionError) undefined function: User.fetch/2
is_map john #=> true
john.__struct__ #=> User
Map.keys(john) #=> [:__struct__, :age, :name]

Ranges

Ranges are Struct.

• range = 1..10 => range definition
• Enum.reduce(1..3, 0, fn i, acc -> i + acc end) #=> 6 => range used in a reduce function to sum them up
• Enum.count(range) #=> 10
• Enum.member?(range, 11) #=> false

Protocols

• defprotocol Foo => define protocol Foo
• defimpl Foo, for Integer => implement that protocol for Integer

Here are all native data types that you can use: Atom, BitString, Float, Function, Integer, List, Map, PID, Port, Reference, Tuple.

defprotocol Blank do
  @doc "Returns true if data is considered blank/empty"
  def blank?(data)
end

defimpl Blank, for: Integer do
  def blank?(_), do: false
end

defimpl Blank, for: List do
  def blank?([]), do: true
  def blank?(_),  do: false
end

defimpl Blank, for: Map do
  def blank?(map), do: map_size(map) == 0
end

defimpl Blank, for: Atom do
  def blank?(false), do: true
  def blank?(nil),   do: true
  def blank?(_),     do: false
end

Blank.blank?(0) #=> false
Blank.blank?([]) #=> true
Blank.blank?([1, 2, 3]) #=> false
Blank.blank?("hello") #=> ** (Protocol.UndefinedError)

Structs do not share Protocol implementations with Map.

defimpl Blank, for: User do
  def blank?(_), do: false
end

You can also implement a Protocol for Any. And in this case you can derive any Struct.

defimpl Blank, for: Any do
  def blank?(_), do: false
end

defmodule DeriveUser do
  @derive Blank
  defstruct name: "john", age: 27
end

Elixir built-in most common used protocols: Enumerable, String.Chars, Inspect.

Nested data Structures

• put_in/2
• update_in/2
• get_and_update_in/2

users = [
  john: %{name: "John", age: 27, languages: ["Erlang", "Ruby", "Elixir"]},
  mary: %{name: "Mary", age: 29, languages: ["Elixir", "F#", "Clojure"]}
]
users[:john].age #=> 27

users = put_in users[:john].age, 31
users = update_in users[:mary].languages, &List.delete(&1, "Clojure")

Enums and Streams

Lists and Maps are Enumerables.

Enum module perform eager operations, meanwhile Stream module perform lazy operations.

# eager Enum
1..100 |> Enum.map(&(&1 * 3)) |> Enum.sum #=> 15150

# lazy Stream
1..100 |> Stream.map(&(&1 * 3)) |> Enum.sum #=> 15150

do/end Keyword List and Block Syntax

In Elixir you can use either Keyword List syntax or do/end Block syntax:

sky = :gray

if sky == :blue do
  :sunny
else
  :cloudy
end

if sky == :blue, do: :sunny, else: :cloudy

if sky == :blue, do: (
  :sunny
), else: (
  :cloudy
)

Conditional Flows (if/else/case/cond)

if / else

sky = :gray
if sky == :blue, do: :sunny, else: :cloudy

unless / else

sky = :gray
unless sky != :blue, do: :sunny, else: :cloudy

case / when

sky = {:gray, 75}
case sky, do: (
  {:blue, _}         -> :sunny
  {_, t} when t > 80 -> :hot
  _                  -> :check_wheather_channel
)

On when guards short-circuiting operators &&, || and ! are not allowed.

cond

cond is equivalent as if/else-if/else statements.

sky = :gray
cond do: (
  sky == :blue -> :sunny
  true         -> :cloudy
)

The with macro

• with => macro to combine multiple match clauses
• <- => a matching clause, on the left
• = => bare expression is allowed
• else => if some matching clause fails

opts = %{width: 10, height: 20}
with {:ok, width} <- Map.fetch(opts, :width),
     {:ok, height} <- Map.fetch(opts, :height) do
  {:ok, width * height}
else
  :error ->
    {:error, :wrong_data}
end
#=> {:ok, 200}

Recursion

There is traditional no for loop in Elixir, due to Elixir immutability There is a macro for that it's also called as Comprehension but it works differently from a traditional for loop. If you want a simple loop iteration you'll need to use recursion:

defmodule Logger do
  def log(msg, n) when n <= 0, do: ()
  def log(msg, n) do
    IO.puts msg
    log(msg, n - 1)
  end
end
Logger.log("Hello World!", 3)
# Hello World!
# Hello World!
# Hello World!

In functional programming languages map and reduce are two major algorithm concepts. They can be implemented with recursion or using the Enum module.

reduce will reduces the array into a single element:

defmodule Math do
  def sum_list(list, sum \\ 0)
  def sum_list([], sum), do: sum
  def sum_list([head | tail], sum) do
    sum_list(tail, head + sum)
  end
end
Math.sum_list([1, 2, 3]) #=> 6

Enum.reduce([1, 2, 3], 0, &+/2) #=> 6

map modifies an existing array (new array with new modified values):

defmodule Math do
  def double([]), do: []
  def double([head | tail]) do
    [head * 2 | double(tail)]
  end
end
Math.double([1, 2, 3]) #=> [2, 4, 6]

Enum.map([1, 2, 3], &(&1 * 2)) #=> [2, 4, 6]

Comprehension -> the for loop

Comprehension is a syntax sugar for the very powerful for special form. You can have generators and filters in that.

• for => Comprehension
• -> => generators
• :into => change return to another Collectable type

You can iterate over Enumerable what makes so close to a regular for loop on other languages:

for n <- [1, 2, 3, 4], do: n * n
#=> [1, 4, 9, 16]

You can also iterate over multiple Enumerable and then create a combination between them:

for i <- [:a, :b, :c], j <- [1, 2], do:  {i, j}
#=> [a: 1, a: 2, b: 1, b: 2, c: 1, c: 2]

You can pattern match each element:

values = [good: 1, good: 2, bad: 3, good: 4]
for {:good, n} <- values, do: n * n
#=> [1, 4, 16]

Generators use -> and they have on the right an Enumerable and on the left a pattern matchable element variable.

You can have filters to filter truthy elements:

for dir  <- [".", "/"],
    file <- File.ls!(dir),
    path = Path.join(dir, file),
    File.regular?(path) do
  "dir=#{dir}, file=#{file}, path=#{path}"
end
#=> ["dir=., file=README.md, path=./README.md", "dir=/, file=.DS_Store, path=/.DS_Store"]

You can use :into to change the return type:

for k <- [:foo, :bar], v <- 1..5, into: %{}, do: {k, v}
#=> %{bar: 5, foo: 5}
for k <- [:foo, :bar], v <- 1..5, into: [], do: {k, v}
#=> [foo: 1, foo: 2, foo: 3, foo: 4, foo: 5, bar: 1, bar: 2, bar: 3, bar: 4, bar: 5]

Anonymous Functions

• fn => define functions
• -> => one line function definition
• . => call a function
• when => guards

add = fn a, b -> a + b end
add.(4, 5) #=> 9

We can have multiple clauses and guards inside functions.

calc = fn
  x, y when x > 0 -> x + y
  x, y -> x * y
end
calc.(-1, 6) #=> 5
calc.(4, 5) #=> 45

Modules And Named Functions

• defmodule => define Modules
• def => define functions inside Modules
• defp => define private functions inside Modules
• when => guards
• @ => module attributes
• __info__(:functions) => list functions inside a module
• defdelegate => delegate functions

defmodule Math do
  def sum(a, b) when is_integer(a) and is_integer(b), do: a + b
end

Math.sum(1, 2) #=> 3

Math.__info__(:functions) #=> [sum: 2]

Module attribute works as constants, evaluated at compilation time:

defmodule Math do
  @foo :bar
  def print, do: @foo
end

Math.print #=> :bar

Special Module attributes:

• @vsn
• @moduledoc
• @doc
• @behaviour
• @before_compile

Default Argument

• \\ => default argument

defmodule Concat do
  def join(a, b, sep \\ " ") do
    a <> sep <> b
  end
end

IO.puts Concat.join("Hello", "world")      #=> Hello world
IO.puts Concat.join("Hello", "world", "_") #=> Hello_world

Default values are evaluated runtime.

defmodule DefaultTest do
  def dowork(x \\ IO.puts "hello") do
    x
  end
end
DefaultTest.dowork #=> :ok
# hello
DefaultTest.dowork 123 #=> 123
DefaultTest.dowork #=> :ok
# hello

Function with multiple clauses and a default value requires a function head where default values are set:

defmodule Concat do
  def join(a, b \\ nil, sep \\ " ") # head function

  def join(a, b, _sep) when is_nil(b) do
    a
  end

  def join(a, b, sep) do
    a <> sep <> b
  end
end

IO.puts Concat.join("Hello")               #=> Hello
IO.puts Concat.join("Hello", "world")      #=> Hello world
IO.puts Concat.join("Hello", "world", "_") #=> Hello_world

Function Capturing

• & => function capturing
• &1 => 1st argument

& could be used with a module function.

When capturing you can use the function/operator with its arity.

&(&1 * &2).(3, 4) #=> 12
(&*/2).(3, 4) #=> 12

(&Kernel.is_atom(&1)).(:foo) #=> true
(&Kernel.is_atom/1).(:foo) #=> true
(&{:ok, [&1]}).(:foo) #=> {:ok, [:foo, :bar]}
(&[&1, &2]).(:foo, :bar) #=> [:foo, :bar]
(&[&1 | [&2]]).(:foo, :bar) #=> [:foo, :bar]

Behaviours

Behaviour modules defines functions

• @callback => defines a function to be implemented by other modules
• :: => separates the function definition to its return type

defmodule Parser do
  @callback parse(String.t) :: any
  @callback extensions() :: [String.t]
end

defmodule JSONParser do
  @behaviour Parser

  def parse(str), do: # ... parse JSON
  def extensions, do: ["json"]
end

Exceptions/Errors => raise/try/rescue

Exceptions/Errors in Elixir are Structs.

• raise "oops" #=> ** (RuntimeError) oops => raises error with message
• raise ArgumentError #=> ** (ArgumentError) argument error => raises an error by module
• raise ArgumentError, message: "oops" #=> ** (ArgumentError) oops => raises an error by module with message
• defexception => define an exception
• try/rescue => catches an error
• throw/try/catch => can be used as circuit breaking, but should be avoided
• exit("my reason") => exiting current process
• after => ensures some resource is cleaned up even if an exception was raised

defmodule MyError do
  defexception message: "default message"
end

is_map %MyError{} #=> true
Map.keys %MyError{} #=> [:__exception__, :__struct__, :message]

raise MyError #=> ** (MyError) default message
raise MyError, message: "custom message" #=> ** (MyError) custom message

You can rescue an error with:

try do
  raise "oops"
rescue
  e in RuntimeError -> e
after
  IO.puts "I can do some clean up here"
end
#=> %RuntimeError{message: "oops"}

try do
  raise "oops"
rescue
  RuntimeError -> "Error!"
end
#=> "Error!"

throw/catch sometime is used for circuit breaking, but you can usually use another better way:

try do
  Enum.each -50..50, fn(x) ->
    if rem(x, 13) == 0, do: throw(x)
  end
  "Got nothing"
catch
  x -> "Got #{x}"
end
#=> "Got -39"

Enum.find -50..50, &(rem(&1, 13) == 0)
#=> -39

exit can be caught but this is rare in Elixir:

try do
  exit "I am exiting"
catch
  :exit, _ -> "not really"
end
#=> "not really"

You can ommit try inside functions and use rescue, catch, after directly:

def without_even_trying do
  raise "oops"
after
  IO.puts "cleaning up!"
end

IO

• IO.puts/1 "Hello" => prints to stdout
• IO.puts :stderr, "Hello" => prints to stderr
• IO.gets "yes/no: " => reads an user input

StringIO

• {:ok, pid} = StringIO.open("my-file.md") => open a file
• IO.read(pid, 2) #=> "he" => read first 2 lines

File

• {:ok, file} = File.open "hello", [:write] => open file for reading
• IO.binwrite file, "world" => write into file
• File.close file => close file
• File.read "my-file.md" => reads a file
• File.stream!("my-file.md") |> Enum.take(10) => read the first 10 lines

{:ok, file} = File.open "my-file.md", [:write]
IO.binwrite file, "hello world"
File.close file
File.read "my-file.md" #=> {:ok, "hello world"}

Path

• Path.join => joins
• Path.expand("~/hello") => expands to full path

Processes, Tasks and Agents

Process in Elixir has the same concept as threads in a lot of other languages, but extremely lightweight in terms of memory and CPU. They are isolated from each other and communicate via message passing.

• spawn/1 => fork a process
• self() => current process
• Process.alive?(pid) => check if process is still running
• send/2 => send message to another process
• receive/1 => receive message from another process
• after => receive option to work with timeout
• flush() => prints out all messages received
• spawn_link/1 => forks a process and link them, so failures are propagated
• Task.start/1 => starts a task
• Task.start_link/1 => starts a task and link it to the current process
• Process.register(pid, :foo) => register a name for a process

The idea is to have a supervisor that spawn_link new processes and when they fail the supervisor will restart them. This is the basics for Fail Fast and Fault Tolerant in Elixir.

Tasks are used in supervision trees.

parent = self()

spawn_link(fn -> send(parent, {:hello, self()}) end)
receive do: ({msg, pid} -> "#{inspect pid} => #{msg}"), after: (1_000 -> "nothing after 1s")

Task.start_link(fn -> send(parent, {:hello, self()}) end)
receive do: ({msg, pid} -> "#{inspect pid} => #{msg}"), after: (1_000 -> "nothing after 1s")

flush()

State can be stored in processes or using its abstraction: Agent.

Manual implementation of a storage using Elixir processes:

defmodule KV do
  def start_link do
    Task.start_link(fn -> loop(%{}) end)
  end

  defp loop(map) do
    receive do
      {:get, key, caller} ->
        send caller, Map.get(map, key)
        loop(map)
      {:put, key, value} ->
        loop(Map.put(map, key, value))
    end
  end
end
{:ok, pid} = KV.start_link

send pid, {:put, :hello, :world}
send pid, {:get, :hello, self()}
flush() #=> :world

Implementation of a storage using Agent:

{:ok, pid} = Agent.start_link(fn -> %{} end)
Agent.update(pid, fn map -> Map.put(map, :hello, :world) end)
Agent.get(pid, fn map -> Map.get(map, :hello) end)

alias, require, import and use

In order to facilitate code reuse Elixir has: alias, require, import (directives) and use (macro).

• alias Foo.Bar, as: Bar => alias module, so Bar can be called instead of Foo.Bar
• alias Foo.Bar => as is optional on alias
• require Foo => ensure the module is compiled and available (usually for macros)
• import Foo => requires and import functions from Foo so they can be called without the Foo. prefix
• import List, only: [duplicate: 2] => only option
• import List, expect: [duplicate: 2] => except option
• import List, only: :macros => import only macros
• import List, only: :functions => import only functions
• use Foo => invokes the custom code defined in Foo as an extension point
• alias MyApp.{Foo, Bar, Baz} => multiple alias
• require MyApp.{Foo, Bar, Baz} => multiple require
• import MyApp.{Foo, Bar, Baz} => multiple import

All modules are defines inside Elixir namespace but it can be omitted for convenience.

alias, require and import are lexically scoped, which means that it will be valid just inside the scope it was defined. This is not a global scope.

require is usually used to require Elixir macro code:

Integer.is_odd(3) #=> ** (CompileError): you must require Integer before invoking the macro Integer.is_odd/1
require Integer
Integer.is_odd(3) #=> true

use call __using__ when the module is being used:

defmodule Fruit do
  defmacro __using__(option: option) do
    IO.puts "options=#{inspect option}"
    quote do: IO.puts "Using Fruit module"
  end
end

defmodule Meal do
  use Fruit, option: :hello
end
#=> "Good to see you've added Fruit to your meal"

Meta Programming

• quote => shows AST (Abstract Syntax Tree)

quote do: 2 * 2 == 4
#=> {
#=>   :==,
#=>   [context: Elixir, import: Kernel],
#=>   [
#=>     {
#=>       :*,
#=>       [context: Elixir, import: Kernel],
#=>       [2, 2]
#=>     },
#=>     4
#=>   ]
#=> }

Erlang libraries

Elixir provider some Erlang modules as atoms.

• :crypto => crypto functions like :crypto.hash/2
• :io => io functions like :io.format/2
• :digraph => deal with digraphs
• :ets => large data structure in memory
• :dets => large data structure on disk
• :math => math functions like :math.pi/0
• :queue => first-in first-out structure
• :rand => rand functions like :rand.uniform/0
• :zip => handle zip files
• :zlib => handle gzip files