Elixir Recipes

Iterating over all keys of the map while keeping key order

Tue, 13 Feb 2018 00:00:00 +0800

Map in Elixir does not guarantee the key sorting order.

# With integer as a key, the key is ordered for the first 32 keys.
iex(1)> map = %{
  1 => 1, 2 => 2, 3 => 3, 4 => 4, 5 => 5, 6 => 6, 7 => 7,
  21 => 1, 22 => 2, 23 => 3, 24 => 4, 25 => 5, 26 => 6, 27 => 7,
  11 => 1, 12 => 2, 13 => 3, 14 => 4, 15 => 5, 16 => 6, 17 => 7,
  61 => 1, 62 => 2, 63 => 3, 64 => 4, 65 => 5, 66 => 6, 67 => 7,
  51 => 1, 52 => 2, 53 => 3, 54 => 4
}

# However, if the number of key is greater than 32, they will not be ordered.
iex(2)> map2 = %{
  1 => 1, 2 => 2, 3 => 3, 4 => 4, 5 => 5, 6 => 6, 7 => 7,
  21 => 1, 22 => 2, 23 => 3, 24 => 4, 25 => 5, 26 => 6, 27 => 7,
  11 => 1, 12 => 2, 13 => 3, 14 => 4, 15 => 5, 16 => 6, 17 => 7,
  61 => 1, 62 => 2, 63 => 3, 64 => 4, 65 => 5, 66 => 6, 67 => 7,
  51 => 1, 52 => 2, 53 => 3, 54 => 4, 55 => 5, 56 => 6, 57 => 7,
  41 => 1, 42 => 2, 43 => 3, 44 => 4, 45 => 5, 46 => 6, 47 => 7
}

Sometime you may want to iterate through the map while keeping the key in order, this could be done by converting it to the list.

iex(3)> sorted_map = Enum.to_list(map2) |> Enum.sort(fn({key1, value1}, {key2, value2}) -> key1 < key2 end)
# [{1, 1}, {2, 2}, {3, 3}, {4, 4}, {5, 5}, {6, 6}, {7, 7}, {11, 1}, {12, 2},
 {13, 3}, {14, 4}, {15, 5}, {16, 6}, {17, 7}, {21, 1}, {22, 2}, {23, 3},
 {24, 4}, {25, 5}, {26, 6}, {27, 7}, {41, 1}, {42, 2}, {43, 3}, {44, 4},
 {45, 5}, {46, 6}, {47, 7}, {51, 1}, {52, 2}, {53, 3}, {54, 4}, {55, 5},
 {56, 6}, {57, 7}, {61, 1}, {62, 2}, {63, 3}, {64, 4}, {65, 5}, {66, 6},
 {67, 7}]

How to connect and debug Elixir or Phoenix App at Heroku?

Sat, 21 Oct 2017 00:00:00 +0800

Heroku has a great feature called heroku ps:exec which allows you to connect to running nodes. You can use this command to connect your elixir nodes easily. And debug your nodes like in your network.

Step 1

Let’s start with Procfile To run a named app at heroku your procfile should specify the app name like:

web: MIX_ENV=prod elixir --sname coolelixirapp -S mix run --no-halt

If you are using Phoenix framework:

web: MIX_ENV=prod elixir --sname coolelixirapp -S mix phx.server

Step 2

Deploy your app with one of these Procfiles depending on your app type.

Step 3

On your local:

$ heroku config # list your heroku app configs
$ heroku ps:exec # --dyno=web.1 # will connect you to web.1 dyno

Note: heroku ps:exec command does not export your configs to the heroku bash shell. So you will need to export them by yourself.

Step 4

Now you are in Heroku bash shell for web.1 dyno:

$ # You may need to export env vars which listed in previous 'heroku config' execution.
$ export DATABASE_URL=postgres://username:password@hostname:5432/dbname;
$ export DATABASE_POOL_SIZE=1;
$ # .... Do your all necessary exports
$ MIX_ENV=prod iex --sname coolelixirconsole -S mix # Run a console app

Step 5

Now you are in Elixir console with app name coolelixirconsole:

>> iex(coolelixirconsole@hostname)1> Node.ping :"coolelixirapp@hostname"
>>> :pong # We are connected to our running Elixir node from heroku ps:exec console
>> iex(coolelixirconsole@hostname)2> Node.list
>>> [:"coolelixirapp@hostname"] # As expected it is in the list
>> iex(coolelixirconsole@hostname)3> # Now you can tell what to do to your app as in your network.

References

https://devcenter.heroku.com/articles/exec
https://hexdocs.pm/phoenix/heroku.html
http://elixir-recipes.github.io/concurrency/connecting-nodes-different-machines/
https://gist.github.com/mustafaturan/de39ecab9f344c91f495f0f51748f7a1 (Original)

Create an array containing the alphabet

Sun, 20 Nov 2016 00:00:00 +0800

A char list containing the alphabet can be obtained by doing the following:

iex> for n <- ?a..?z, do: n
'abcdefghijklmnopqrstuvwxyz'

If desired, we can pass each bitcode to a binary to represent each letter as a string:

iex> for n <- ?a..?z, do: << n :: utf8 >>
["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p",
"q", "r", "s", "t", "u", "v", "w", "x", "y", "z"]

Additional reading:

Binaries, strings, and char lists

Cancel a multiline command

Sun, 20 Nov 2016 00:00:00 +0800

When typing multiline commands into IEx it is easy to get lost in opening and closing symbols. For those cases you can issue #iex:break to interrupt the declaration of that command. For example:

iex> IO.puts "hello world
...>
...> #iex:break
** (TokenMissingError) iex:1: incomplete expression
iex>

Additional reading:

Elixir interactive shell

Finding matching elements in a list or a map

Thu, 17 Nov 2016 00:00:00 +0800

When storing data in lists or maps in Elixir we can use pattern matching to access the elements of those collections easily.

Below are a few ways of how to do it.

defmodule Find do
  @moduledoc """
  Implements methods to find elements in given collections by pattern matching.
  """

  @doc """
  Finds the first element in a list to match a given pattern.
  """
  def first_match(collection) do
    Enum.find(collection, fn(element) ->
      match?({:fruit, _}, element)
    end)
  end

  @doc """
  Finds all the elements in a list that match a given pattern.
  """
  def all_matches(collection) do
    Enum.filter(collection, fn(element) ->
      match?({:fruit, _}, element)
    end)
  end
end

And then, to test the implemented methods:

iex> require Find
iex> array = [{:fruit, "Apple"}, {:vegetable, "Carrot"}, {:fruit, "Orange"}]
iex> Find.first_match(array)
{:fruit, "Apple"}
iex> Find.all_matches(array)
[{:fruit, "Apple"}, {:fruit, "Orange"}]
iex> map = %{fruit: "Apple", vegetable: "Carrot"}
iex> Find.first_match(map)
{:fruit, "Apple"}
iex> Find.all_matches(map)
[{:fruit, "Apple"}]

Additional reading:

Counting words in a file

Fri, 28 Oct 2016 00:00:00 +0800

Use the File module to read a file.

file = "foo.txt"

case File.read(file) do
  {:ok, body} ->
    body
    |> IO.puts
  {:error, reason} ->
    "Could not read file #{file} because #{reason}"
    |> IO.puts
end

One application of this could be to count the number of words in a file.

file = "foo.txt"

case File.read(file) do
  {:ok, body} ->
    String.split(body)
    |> length
    |> IO.puts
  {:error, reason} ->
    "Could not read file #{file} because #{reason}"
    |> IO.puts
end

Checking if a string includes a substring

Wed, 19 Oct 2016 00:00:00 +0800

Use the =~ operator to check if a string on the left-hand-side includes a substring on the right-hand-side:

iex> "1test2" == "test"
false
iex> "1test2" =~ "test"
true

One application of =~ in Phoenix is in controller tests:

defmodule RealtimeUsers.PageControllerTest do
  use RealtimeUsers.ConnCase

  test "GET /", %{conn: conn} do
    conn = get conn, "/"
    assert html_response(conn, 200) =~ "Welcome to Phoenix!"
  end
end

List all Phoenix routes

Thu, 29 Sep 2016 00:00:00 +0800

Use the mix phoenix.routes to list out all your routes:

> mix phoenix.routes
page_path  GET  /                  RealtimeUsers.PageController :index
hello_path  GET  /hello             RealtimeUsers.HelloController :index
hello_path  GET  /hello/:messenger  RealtimeUsers.HelloController :show

Monitoring a process

Sun, 11 Sep 2016 00:00:00 +0800

Monitoring a process notifies the monitor process if the monitored process crashes. Unlike spawn_link, it doesn’t crash the monitoring process:

# Span a new process
pid = spawn(fn -> :timer.sleep(100) end)

# Monitor the process
Process.monitor(pid)

# Kill the process
Process.exit(pid, :kill)

# Flush the messages of the inbox
flush
{:DOWN, #Reference<0.0.1.82>, :process, #PID<0.89.0>, :killed}

Convert atoms to strings

Fri, 02 Sep 2016 00:00:00 +0800

Use Atom.to_string:

iex> Atom.to_string(:foo)
"foo"