Elixir

Baby steps

• Install using this guide: https://elixir-lang.org/getting-started/introduction.html#installation
• Verify if the installation is done: $elixir --version
• Run interactive shell: $iex
• Execute a source file: $elixir abc.exs

Mix

• mix is a the Elixir build tool:
  • To create a new project from scaffolding: $ mix new {folder_name} --module {module_name}
  • To compile a project, go into the project folder and: $ mix compile
  • Once project has been compiled, you can start a session inside: $ iex -S mix
    • You can do live debugging here, e.g. KV.hello
  • To run test, go into the project folder and: $ mix test
  • $ mix help to have more information
  • mix deps.get to install all dependencies (Ruby's bundle install)
• Get help in iex:
  • h <module_name> to get help, e.g. h Map
  • h <func> to get information about the function, e.g. h Map.keys/1
  • i variable_name to get information about that variable

Syntax

• IO.puts "Hello world" to print onto console
• Process.sleep(10) to put the process to sleep for 10 millisec
• pid = spawn(ModuleName, :method, []) to spawn a new process
• send pid, message to send message to the process
• flush to flush all message to shell
• :observer.start to start the ErlangVM' observer app
• IEx.Helpers.recompile to reload code from inside iex

Clustering

• iex --sname quynh to start a new shell with (short) name = quynh

• iex --name [email protected] to start a new shell with a long name

• From the shell, i.e. iex

  • node to get the name of node name of the VM
  • Node.list to get the list nodes that connect to the current one
  • Node.connect <name> to connect the current shell to another node

• :rpc.multical/3 to execute code in the cluster

• mix escript.build to generate the command line

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Postgres

• Start the service: pg_ctl -D /usr/local/var/postgres start
• Stop the service: pg_ctl -D /usr/local/var/postgres stop
• Version: postgres -V
• Command line: psql postgres
  • \du: see what's installed on the machine
  • \l: list of databases
  • \d: list of tables

GenServer

Supervisor

• Process.info(self, :links) to find all processes linked to the current one
• Process.link(pid) to link the current process to the pid one
• Process.alive?(pid) whether a process is still alive
• Process.flag(:trap_exit, true) to trap the exit signals from the current process

Ring project - Chapter 5

Ring of linked process:

• iex -S mix: Get into interactive console
• pids = Ring.create_processes(10): Create 10 processes
• Ring.link_processes(pids): Create a ring of linked processes
• pids |> Enum.map(fn pid -> "#{inspect pid}: #{inspect Process.info(pid, :links)}" end): Inspec the ring
• pids |> Enum.shuffle |> List.first |> send(:crash): Crash a random process in the ring
• pids |> Enum.map(fn pid -> Process.alive?(pid) end): Check whether any process is still alive

Trapping exits:

• Process.flag(:trap_exit, true): Trap the exit signals from the current process
• pid = spawn(fn -> receive do :crash -> 1/0 end end): Create a process that will crash on signal
• Process.link(pid): Link two processes
• send(pid, :crash): Crash the target process
• flush: To see the crash message the current process receive
• Note that the current process still survice after the crash

3 ways for a process to terminate:

• Naturally, no more code to run

  • pid = spawn(fn -> receive do :ok -> :ok end end): a process to wait for only one message
  • Process.link(pid)
  • send(pid, :ok): execute the code
  • flush: to see the message on the shell

• Forcefully terminated

  • Process.flag(:trap_exit, true): trap the exit signals
  • Process.exit(self, :whoops): raise exit signal
  • flush: to see the exit signal was trapped
  • Process.exit(self, **:kill**): send an untrappable signal, the shell will be crashed

• Exception on a process, per the previous example

Data types

Elixir support the following data types:

• Value types
  • Integer
  • Float
  • Atom
  • Range
  • Regex
• System types
  • PID and port
  • Reference
• Collection types
  • Tuple
  • List
  • Maps
  • Binaries

Utilities:

• i command in iex: Print out information about a variable

Plan

• Blitzy command-line program
• Registry for message dispatching & pub/sub

Phoenix

• Install the latest version of Phoenix: mix archive.install https://github.com/phoenixframework/archives/raw/master/phx_new.ez
• mix phx.new to create a new project, e.g. mix phx.new conduit --module Conduit --app conduit --no-brunch --no-html
• mix phx.server to run the Phoenix webserver, default URL: http://localhost:4000/
• mix phx.routes to show the defined routes in the application

##Dev Environment

• Erlang version: erl -eval 'erlang:display(erlang:system_info(otp_release)), halt().' -noshell
• Elixir version: elixir -v
• asdf for version manager:
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