Elixir cheatsheet

Getting started

Hello world

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

Greeter.greet("world")

elixir hello.exs
# Hello, world!

Variables

age = 23

Maps

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

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

Lists

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

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

Piping

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

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

These two are equivalent.

Pattern matching

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

This sets username to "Tom".

Pattern matching in functions

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

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

Pattern matching works in function parameters too.

Control flow

If

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

Case

case {1, 2, 3} do
  {4, 5, 6} ->
    "This clause won't match"
  {1, x, 3} ->
    "This will match and bind x to 2"
  _ ->
   "This will match any value"
end

Cond

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

With

with {:ok, {int, _asdf}} <- Integer.parse("123asdf"),
     {:ok, datetime, _utc_offset} <- DateTime.from_iso8601("2021-10-27T12:00:00Z") do
  DateTime.add(datetime, int, :second)
  # optional else clause. if not provided and an error occurs, the error is returned
else
  :error -> "couldn't parse integer string"
  {:error, :invalid_format} -> "couldn't parse date string"
  _ -> "this will never get hit because all errors are handled"
end

Errors

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

Types

Primitives

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()

value |> IO.inspect(label: "value")

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_charlist()    # → '12'
n |> to_string()      # → "12"
n |> is_even()
n |> is_odd()

# Different base:
n |> digits(2)        # → [1, 1, 0, 0]
n |> to_charlist(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

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

Updating

import Map

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

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

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

m |> get_and_update(:a, &(&1 || "default"))
# → {old, new}

Deleting

m |> delete(:name)  # → %{}
m |> pop(:name)     # → {"John", %{}}

Reading

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

m |> 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 from lists

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}

Working with structs

Struct to map

Map.from_struct(%AnyStruct{a: "b"})  # → %{a: "b"}

Map to struct

struct(AnyStruct, %{a: "b"})  # → %AnyStruct{a: "b"}

List

import List

l = [ 1, 2, 3, 4 ]

l = l ++ [5]         # push (append)
l = [ 0 | list ]     # unshift (prepend)

l |> first()
l |> last()

l |> flatten()
l |> flatten(tail)

Also see Enum.

Enum

Usage

import Enum

list = [:a, :b, :c]

list |> at(0)         # → :a
list |> count()       # → 3
list |> empty?()      # → false
list |> any?()        # → true

list |> concat([:d])  # → [:a, :b, :c, :d]

Also, consider streams instead.

Map/reduce

list |> reduce(fn)
list |> reduce(acc, fn)
list |> map(fn)
list |> reject(fn)
list |> any?(fn)
list |> empty?(fn)

[1, 2, 3, 4]
|> Enum.reduce(0, fn(x, acc) -> x + acc end)

Tuple

Tuples

import Tuple

t = { :a, :b }

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

Keyword lists

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

# For string-keyed 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(charlist)

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

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