cheatsheets/elixir.md

title	category	layout	tags	updated	weight
Elixir	Elixir	2017/sheet	New	201708	-10

Getting started

{: .-three-column}

Hello world

{: .-prime}

# hello.exs
defmodule Greeter do
  def greet(name) do
    IO.puts "Hello, " <> name <> "!"
  end
end

Greeter.greet("world")

elixir hello.exs
# Hello, world!

{: .-setup}

Variables

age = 23

Maps

user = %{
  name: "John",
  city: "Melbourne"
}

IO.puts "Hello, " <> user.name

{: .-setup}

Lists

users = [ "Tom", "Dick", "Harry" ]

{: data-line="1"}

Enum.map(user, fn user ->
  IO.puts "Hello " <> user
end)

Piping

source
|> transform(:hello)
|> print()

{: data-line="2,3"}

# Same as:
print(transform(source, :hello))

These two are equivalent.

Pattern matching

user = %{name: "Tom", age: 23}
%{name: username} = user

{: data-line="2"}

This sets username to "Tom".

Pattern matching in functions

def greet(%{name: username}) do
  IO.puts "Hello, " <> username
end

user = %{name: "Tom", age: 23}

{: data-line="1"}

Pattern matching works in function parameters too.

Control flow

{: .-three-column}

If

if false do
  "This will never be seen"
else
  "This will"
end

Cond

cond do
  1 + 1 == 3 ->
    "I will never be seen"
  2 * 5 == 12 ->
    "Me neither"
  _ ->
    "But I will (this is essentially an else)"
end

Errors

try do
  throw(:hello)
catch
  message -> "Got #{message}."
after
  IO.puts("I'm the after clause.")
end

Types

Primitives

Sample	Type
nil	Nil/null
true / false	Boolean
---	---
?a	Integer (ASCII)
23	Integer
3.14	Float
---	---
'hello'	Char list
<<2, 3>>	Binary
"hello"	Binary string
:hello	Atom
---	---
[a, b]	List
{a, b}	Tuple
---	---
%{a: "hello"}	Map
%MyStruct{a: "hello"}	Struct
fn -> ... end	Function

Type checks

is_atom/1
is_bitstring/1
is_boolean/1
is_function/1
is_function/2
is_integer/1
is_float/1

is_binary/1
is_list/1
is_map/1
is_tuple/1

is_nil/1
is_number/1
is_pid/1
is_port/1
is_reference/1

Operators

left != right   # equal
left !== right  # match
left ++ right   # concat lists
left <> right   # concat string/binary
left =~ right   # regexp

Modules

Importing

require Redux   # compiles a module
import Redux    # compiles, and you can use without the `Redux.` prefix

use Redux       # compiles, and runs Redux.__using__/1
use Redux, async: true

import Redux, only: [duplicate: 2]
import Redux, only: :functions
import Redux, only: :macros

import Foo.{Bar, Baz}

Aliases

alias Foo.Bar, as: Bar
alias Foo.Bar   # same as above

alias Foo.{Bar, Baz}

String

Functions

import String

str = "hello"
str |> length()      #=> 5
str |> codepoints()  #=> ["h", "e", "l", "l", "o"]
str |> slice(2..-1)  #=> "llo"
str |> split(" ")    #=> ["hello"]
str |> capitalize()  #=> "Hello"
str |> match(regex)

Inspecting objects

inspect(object, opts \\ [])

value |> IO.inspect()

Numbers

Operations

abs(n)
round(n)
rem(a, b)   # remainder (modulo)
div(a, b)   # integer division

Float

import Float

n = 10.3
n |> ceil()            #=> 11.0
n |> ceil(2)           #=> 11.30
n |> to_string()       #=> "1.030000+e01"
n |> to_string([decimals: 2, compact: true])

Float.parse("34")  #=> { 34.0, "" }

Integer

import Integer

n = 12
n |> digits()        #=> [1, 2]
n |> to_char_list()  #=> '12'
n |> to_string()     #=> "12"
n |> is_even()
n |> is_odd()

# Different base:
n |> digits(2)        #=> [1, 1, 0, 0]
n |> to_char_list(2)  #=> '1100'
n |> to_string(2)     #=> "1100"

parse("12")       #=> 12
undigits([1, 2])  #=> 12

Type casting

Float.parse("34.1")    #=> {34.1, ""}
Integer.parse("34")    #=> {34, ""}

Float.to_string(34.1)  #=> "3.4100e+01"
Float.to_string(34.1, [decimals: 2, compact: true])  #=> "34.1"

Map

Defining

map = %{name: "hi"}       # atom keys (:name)
map = %{"name" => "hi"}   # string keys ("name")

Updating

import Map

map = %{map | name: "yo"}  # key must exist

map |> put(:id, 2)      #=> %{id: 2, name: "hi"}
map |> put_new(:id, 2)  # only if `id` doesn't exist (`||=`)

map |> put(:b, "Banana")
map |> merge(%{b: "Banana"})
map |> update(:a, &(&1 + 1))
map |> update(:a, fun a -> a + 1 end)

{old, new} = map |> get_and_update(:a, &(&1 || "default"))

Deleting

map |> delete(:name)    #=> "hi"
map |> pop(:name)       #=> %{id: 1}

Reading

map |> get(:id)         #=> 1
map |> keys()           #=> [:id, :name]
map |> values()         #=> [1, "hi"]

map |> to_list()      #=> [id: 1, name: "hi"]
                      #=> [{:id, 1}, {:name, "hi"}]

Deep

put_in(map, [:b, :c], "Banana")
put_in(map[:b][:c], "Banana")    # via macros
get_and_update_in(users, ["john", :age], &{&1, &1 + 1})

Constructing

Map.new([{:b, 1}, {:a, 2}])
Map.new([a: 1, b: 2])
Map.new([:a, :b], fn x -> {x, x} end)  #=> %{a: :a, b: :b}

List

import List
list = [ 1, 2, 3, 4 ]
list = list ++ [5]      # push (append)
list = [ 0 | list ]     # unshift (prepend)

first(list)
last(list)

flatten(list)
flatten(list, tail)

Also see Enum.

Enum

import Enum

# High-order
reduce(list, acc, fn)
map(list, fn)
reject(list, fn)
any?(list, fn)
empty?(list, fn)

list = [:a, :b, :c]
at(list, 0)         #=> :a
count(list)         #=> 3
empty?(list)        #=> false
any?(list)          #=> true

concat(list, [:d])  #=> [:d]

Also, consider streams instead.

Tuples

Tuples

tuple = { :a, :b }

tuple |> elem(1)    # like tuple[1]
tuple |> put_elem(index, value)
tuple |> tuple_size()

Keyword lists

list = [{ :name, "John" }, { :age, 15 }]
list[:name]

# For string-indexed keyword lists
list = [{"size", 2}, {"type", "shoe"}]
List.keyfind(list, "size", 0)  #=> {"size", 2}

Functions

Lambdas

square = fn n -> n*n end
square.(20)

& syntax

square = &(&1 * &1)
square.(20)

square = &Math.square/1

Running

fun.(args)
apply(fun, args)
apply(module, fun, args)

Function heads

def join(a, b \\ nil)
def join(a, b) when is_nil(b) do: a
def join(a, b) do: a <> b

Structs

Structs

defmodule User do
  defstruct name: "", age: nil
end

%User{name: "John", age: 20}

%User{}.struct  #=> User

See: Structs

Protocols

Defining protocols

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

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

Blank.blank?([])  #=> true

Any

defimpl Blank, for: Any do ... end

defmodule User do
  @derive Blank     # Falls back to Any
  defstruct name: ""
end

Examples

• Enumerable and Enum.map()
• Inspect and inspect()

Comprehensions

For

for n <- [1, 2, 3, 4], do: n * n
for n <- 1..4, do: n * n

for {key, val} <- %{a: 10, b: 20}, do: val  #=> [10, 20]
for {key, val} <- %{a: 10, b: 20}, into: %{}, do: {key, val*val}

Conditions

for n <- 1..10, rem(n, 2) == 0, do: n
#=> [2, 4, 6, 8, 10]

Complex

for dir <- dirs,
    file <- File.ls!(dir),          # nested comprehension
    path = Path.join(dir, file),    # invoked
    File.regular?(path) do          # condition
  IO.puts(file)
end

Misc

Metaprogramming

__MODULE__
__MODULE__.__info__

@after_compile __MODULE__
def __before_compile__(env)
def __after_compile__(env, _bytecode)
def __using__(opts)    # invoked on `use`

@on_definition {__MODULE__, :on_def}
def on_def(_env, kind, name, args, guards, body)

@on_load :load_check
def load_check

Regexp

exp = ~r/hello/
exp = ~r/hello/i
"hello world" =~ exp

Sigils

~r/regexp/
~w(list of strings)
~s[strings with #{interpolation} and \x20 escape codes]
~S[no interpolation and no escapes]
~c(char list)

Allowed chars: / | " ' ( [ { < """. See: Sigils

Type specs

@spec round(number) :: integer

@type number_with_remark :: {number, String.t}
@spec add(number, number) :: number_with_remark

Useful for dialyzer. See: Typespecs

Behaviours

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

See: Module

References

{: .-one-column}

• Learn Elixir in Y minutes