Elixir Club 9 November 18, 2017 Kharkiv

1. Александр Хохлов
@apoint
Pattern matching
in Elixir by example

2. Founder at Nots
http://nots.io

3. TL;DR

4. https://images.techhive.com/images/article/2016/06/integration-projects-disasters-9-100669080-gallery.idge.jpg

5. https://twitter.com/bloerwald/status/448415935926255618

6. История

7. Erlang The Movie II: The Sequel
https://www.youtube.com/watch?v=rRbY3TMUcgQ
https://img00.deviantart.net/de6b/i/2011/044/b/9/erlang_the_movie_by_mcandre-d39hupa.png

8. Нет присваивания
(assignment)

10. iex > a = 1
1
iex > 1 = a
1
iex > 2 = a
** (MatchError) no match of right hand side
value: 1

11. Переменные можно связывать
заново (rebind)

12. iex > b = 2
2
iex > b = 3
3
iex > ^b = 4
** (MatchError) no match of right hand side
value: 4
iex > ^b = 3
3

13. Можно сопоставлять
структуры данных

14. iex > {:ok, value} = {:ok, "Successful!"}
{:ok, "Successful!"}
iex > value
“Successful!”
iex > {:ok, value} = {:error, "Shit:("}
** (MatchError) no match of right hand side
value: {:error, “Shit:(“}
iex > {:ok, value} = [:ok, "Success"]
** (MatchError) no match of right hand side
value: [:ok, "Success"]

15. iex > %{key: value} = %{key: “hash value"}
%{key: “hash value"}
iex > value
“hash value”
iex > %{key1: value} = %{key1: "value1", key2: "value2"}
%{key1: "value1", key2: "value2"}
iex > value
“value1"
iex > key = :key1
:key1
iex > %{^key => value} = %{key1: "value1", key2: "value2"}
%{key1: "value1", key2: "value2"}
iex > value
"value1"

16. А еще списки

17. iex > list = [1, 2, 3]
[1, 2, 3]
iex > [1 | tail] = list
[1, 2, 3]
iex > tail
[2, 3]
iex > [2 | _] = list
** (MatchError) no match of right hand side
value: [1, 2, 3]
iex > [head | tail] = list
[1, 2, 3]
iex > head
1

18. Функции

20. iex > defmodule Hello do
... > def hello(name) do
... > "Hello, #{name}"
... > end
... > end
… skipped …
iex > Hello.hello("point")
"Hello, point"

21. iex > defmodule Hello do
... > def hello(:point) do
... > "Greeting, my lord"
... > end
... > def hello(name) do
... > "Hello, #{name}"
... > end
... > end
iex > Hello.hello(:point)
"Greeting, my lord"
iex > Hello.hello("John")
"Hello, John"

22. def hello(:point)
def hello("Alex" <> _)
def hello([name1, name2 | _])
def hello(%{first_name: name})
def hello(_)
def hello(_name)

23. Хардкор

25. iex > defmodule Person do
... > defstruct first_name: "", last_name: ""
... > end
iex > def hello(%Person{} = person) do
... > IO.puts("Hello, #{person.first_name}")
... > end
iex > Hello.hello(%Person{first_name: "Arthur",
last_name: "Dent"})
Hello, Arthur
def hello(%Person{first_name: first_name})

26. def hello(%x{} = person) when x in [Person] do
IO.puts("Hello, #{person.first_name}")
end

27. defmodule Person do
defstruct age: 0
end
defmodule Greeting do
def hello(%{age: age}) when 6 < age and age < 12, do:
"Hiya"
def hello(%{age: age}) when age in 12..18, do:
"Whatever"
def hello(%{age: age}) when 60 < age, do:
“You kids get off my lawn"
def hello(_), do: "Hello"
end
https://hexdocs.pm/elixir/master/guards.html

28. def hello() do
result = case {:ok, "Successful!"} do
{:ok, result} -> result
{:error} -> "Shit:("
_ -> "Catch all"
end
# result == "Successful!"
end

29. defmodule Factorial do
def of(0), do: 1
def of(n) when n > 0 do
n * of(n - 1)
end
end
iex > Factorial.of(10)
3628800

30. defmodule ImageTyper do
@png_signature <<137::size(8), 80::size(8), 78::size(8), 71::size(8),
13::size(8), 10::size(8), 26::size(8), 10::size(8)>>
@jpg_signature <<255::size(8), 216::size(8)>>
def type(<<@png_signature, rest::binary>>), do: :png
def type(<<@jpg_signature, rest::binary>>), do: :jpg
def type(_), do :unknown
end

32. def create(params) do
case validate_name(params["name"]) do
{:ok, name} ->
case validate_email(params["email"]) do
{:ok, email} ->
create_db_record(name, email)
{:error, message} ->
conn |> put_flash(:error, "Wrong email: #{message}")
|> redirect(to: "/")
end
{:error, message} ->
conn |> put_flash(:error, "Wrong name: #{message}")
|> redirect(to: "/")
end
end

33. def create(params) do
with {:ok, name} <- validate_name(params["name"]),
{:ok, email} <- validate_email(params["email"])
do
create_db_record(name, email)
else
{:name_error, message} ->
conn |> put_flash(:error, "Wrong name: #{message}") |>
redirect(to: "/")
{:email_error, message} ->
conn |> put_flash(:error, "Wrong email: #{message}") |
> redirect(to: "/")
end
end

35. defmodule MyAppWeb.PageController do
action_fallback MyAppWeb.FallbackController
def show(params) do
with {:ok, username} <- get_username(params),
{:ok, cms_page} <- CMS.get_page(username, params) do
render(conn, "show.html", page: page)
end
end
end
defmodule MyAppWeb.FallbackController do
def call(conn, {:username_error, message}) do
conn |> put_flash(:error, "Wrong usernname: #{message}") |> redirect(to: "/")
end
def call(conn, {:cms_page_not_found, message}) do
conn |> put_flash(:error, "Page not found: #{message}") |> redirect(to: "/")
end
end

36. defmodule NotsappWeb.ProjectsFallbackController do
use Phoenix.Controller
def call(conn, _) do
conn
|> put_status(:not_found)
|> put_layout(false)
|> render(NotsappWeb.ErrorView, :”501")
end
end

37. @apoint
point@nots.io
http://nots.io/jobs
@nots_io
facebook.com/nots.io