Functionaλ Programming

f(g(x), h(...))

Steve Vinoski, IEEE Computing Society 2009

"... ironically, the resurgence of interest in FP languages owes a lot to Java’s popularity."

"Another reason [...] is the move toward multicore architectures."

"... developers look for the opportunity to
get more done with less."

Welcome to the Functional Web

Functional Programming Languages

Lisp

Elm

Elixir

José Valim

Core committer

Unsatisfied with thread-based concurrency

~2010

Unsatisfied with thread-based concurrency

Repetitive strain

injury

by Bruce A. Tate

So what's interesting about

this Erlang thing?

What even an "Erlang" is?

A bit of History

Erlang

context

Created in the Telecommunication industry

...

~1980

Erlang

requirements

Concurrency
Soft real-time
Distribution
Hardware Interaction
Large Software Systems
Complex Functionality
Continuous Operation
Quality Requirements
Fault Tolerance

Programming language

Virtual Machine - BEAM

Framework - OTP

-module(fact).
-export([fac/1]).

fac(0) -> 1;
fac(N) -> N * fac(N-1).

Cases

The VM

Concurreny

Fault tolerance

Data structures

Consistent libs

Modern tooling

I loved everything that Erlang had

but I hated everything that it didn't had

- José Valim

Elixir

~2011

Elixir

Goals

Extensibility
Productivity
Compatibility

Elixir

Erlang

Features

Pattern Matching

> {method, path} = {:get, "temp"}
{:get, "temp"}
> method
:get
> path
"temp"

b = :binary.encode_unsigned(258)
# b has value <<1, 2>>

<<x, 2>> = b
# x has value 1

i = :binary.decode_unsigned(<<x, 2>>)
# i has value 258

request = {:get, "temp"}



case request do
  {:post, path, payload} ->
    send_resp 200
  {:get, path} ->
    send_resp 200, "{'value':17.5, 'unit': 'celsius'}"
end

defmodule Math do
  def fib(0), do: 0
  def fib(1), do: 1
  def fib(n), do: fib(n-1) + fib(n-2)
end

Math.fib(8) # => 21

Anonymous Functions

> add = fn(a, b) -> a + b end
#Function<12.71889879/2 in :erl_eval.expr/5>
> add.(1, 2)
3

> Enum.map([17.9, 20.2, 18.4], fn(x) -> x * 2 end)
[35.8, 40.4, 36.8]

Can be passed as parameters

Processes

spawn

A lightweight thread of execution

(The key Erlang/Elixir abstraction)

How is this different from threads?

No shared memory
VM Processes

Do not share memory

Communicate via

Message Passing

message

VM Processes

Excuse me, have you ever heard of (VM) processes?

Erlang VM

Scheduler

Sched.

OS Processes

OS Threads

VM Processes

OS

pid = spawn(fn ->
  :timer.sleep(3000)
  IO.puts "hi"
end)

fn ..

spawn

self()

sleep

puts

A lightweight thread of execution

Messages

pid = spawn(...)
send(pid, "hello")

receive do
  msg ->
    IO.puts msg
end

fn ..

spawn

self()

receive

send

Node.spawn(
  :"app@computer-2",
  fn -> Hello.world end)

send

receive

app@computer-1

app@computer-2

Distributed

Same syntax

We have:

Isolated Processes
Message passing
Distribution

We want:

Fault Tolerance

dangling processes!

spawn

spawn

call the supervisor

supervisor

worker

worker

Supervision Tree

import Supervisor.Spec

children = [
  worker(Cache, []),
  supervisor(DatabaseSupervisor, []),
  supervisor(TCPSupervisor, [4040])
]

Supervisor.start_link(children, strategy: :one_for_one)

Applications

your app

http
server

logger

app@computer-1

Distributed Applications

app@computer-2

Fault Tolerance

Open Telecom Platform

Supervisor
Application
GenServer
GenEvent

OOP

Objects

Elixir

Processes

Production-ready?

https://github.com/doomspork/elixir-companies

CoAP

Constrained Application Protocol
kind of a "smaller HTTP, for IoT"

request

response

CoAP - Codes

(binary header)

0.01 (GET) (Request)

0.02 (POST) (Request)

2.05 (Content) (Response)

4.04 (Not Found) (Response)

000.00001

000.00010

010.00101

100.00100

CoAP

parser

1

0

0.01

123

01000000 00000001 0000000001111011 11111111

255

64 1 0 123 255

(empty)

%Message(type: :con,
         code: :get,
         msg_id: 123,
         options: [])

int coap_parseHeader(coap_header_t *hdr, 
                     const uint8_t *buf,
                     size_t buflen)
{
    if (buflen < 4)
        return COAP_ERR_HEADER_TOO_SHORT;
    hdr->ver = (buf[0] & 0xC0) >> 6;
    if (hdr->ver != 1)
        return COAP_ERR_VERSION_NOT_1;
    hdr->t = (buf[0] & 0x30) >> 4;
    hdr->tkl = buf[0] & 0x0F;
    hdr->code = buf[1];
    hdr->id[0] = buf[2];
    hdr->id[1] = buf[3];
    return 0;
}

https://github.com/1248/microcoap/blob/master/coap.c

C

def decode(<<1::2, type::2, token_len::4,
             code_class::3, code_detail::5,
             msg_id::16,
             token::binary-size(token_len),
             rest::binary>>) do

def decode(<<1::2, type::2, token_len::4,
             code_class::3, code_detail::5,
             msg_id::16,
             token::binary-size(token_len),
             rest::binary>>) do
  code = decode_code(code_class, code_detail)
  token = decode_token(token, token_len)
  {options, payload} =
    decode_options_and_payload(rest)

  %Message{
    version: 1,
    type: Registry.from(:types, type),
    code: code,
    msg_id: msg_id, token: token,
    options: options, payload: payload
  }
end

int coap_parseHeader(coap_header_t *hdr, 
                     const uint8_t *buf,
                     size_t buflen)
{
    if (buflen < 4)
        return COAP_ERR_HEADER_TOO_SHORT;
    hdr->ver = (buf[0] & 0xC0) >> 6;
    if (hdr->ver != 1)
        return COAP_ERR_VERSION_NOT_1;
    hdr->t = (buf[0] & 0x30) >> 4;
    hdr->tkl = buf[0] & 0x0F;
    hdr->code = buf[1];
    hdr->id[0] = buf[2];
    hdr->id[1] = buf[3];
    return 0;
}

Hands on!

Can read & parse its own Service Description
Can Register itself in the Broker
Has a REST API
Enforces Access Control
Has unit tests
Can keep state

Goal:

Build a

Temperature Sensor

service that:

#1

Create a new project

$ mix new temperature_sensor --sup

$ cd temperature_sensor

$ mix test

$ iex -S mix

#2

"Can read the service description"

put jsonld file at "priv" dir

def read_service_description do
  Path.join("#{:code.priv_dir(:temperature_sensor)}", 
            "service_description.jsonld")
  |> File.read!()
end

$ mix test

$ iex -S mix

#3

"Add Poison json parser"

# mix.exs

defp deps do
  [{:poison, "~> 3.0"}]
end

def read_service_description do
  Path.join("#{:code.priv_dir(:temperature_sensor)}", 
            "service_description.jsonld")
  |> File.read!()
  |> Poison.decode!()
end

$ mix deps.get

$ mix test

#4

"Add 'register' functionality "

def register(sd) do
  Task.start_link(fn ->
    BrokerHTTPClient.register(sd)
  end)
end

# mix.exs

defp deps do
  [
    {:poison, "~> 3.0"},
    {:broker_http_client, 
      git: "git@git.febrace.org.br:swarm-unit/broker-http-client-ex.git"}
  ]
end

$ mix deps.get

$ mix test

#5

"Turn it into a webserver"

Plug.Adapters.Cowboy.child_spec(:http, 
                                TemperatureSensor.Router, 
                                [], [port: sd_port])

# mix.exs

defp deps do
  [
    {:cowboy, "~> 1.0.0"},
    {:plug, "~> 1.0"},
    {:poison, "~> 3.0"},
    {:broker_http_client,
     git: "git@git.febrace.org.br:swarm-unit/broker-http-client-ex.git"}
  ]
end

$ mix deps.get

$ iex -S mix

#6

"Add tests to the webserver"

defmodule RouterTest

$ mix test

#7

"Add PUT and 404 handlers, and test for 404 path"

match _ do
  Logger.warn "Invalid request at #{conn.request_path}"
  send_resp(conn, 404, "EEEPA")
end

$ mix test

$ iex -S mix

#8

"Add access control verification"

plug BrokerHTTPClient, :authorize

$ iex -S mix

$ curl \
  -H "x-m2m-origin: alice" 
  http://localhost:8001/temperature-sensor/value

Can read & parse its own Service Description
Can Register itself in the Broker
Has a REST API
Enforces Access Control
Has unit tests
Can keep state

Goal:

Build a

Temperature Sensor

service that:

Full code at http://git.febrace.org.br/Geovane/sample_temperature_sensor/

Appendix

:observer.start

Elixir: a practical introduction

Contents

Functionaλ Programming

Welcome to the Functional Web

Functional Programming Languages

Elixir

José Valim

So what's interesting about

this Erlang thing?

What even an "Erlang" is?

A bit of History

Erlang

Erlang

Cases

Elixir

~2011

Elixir

Extensibility

Productivity

Compatibility

Elixir

Erlang

Features

Pattern Matching

Anonymous Functions

Processes

A lightweight thread of execution

How is this different from threads?

No shared memory

VM Processes

Do not share memory

Communicate via

Message Passing

VM Processes

Erlang VM

A lightweight thread of execution

Messages

Distributed

Same syntax

We have:

Isolated Processes

Message passing

Distribution

We want:

Fault Tolerance

dangling processes!

call the supervisor

supervisor

Supervision Tree

Applications

your app

httpserver

logger

Distributed Applications

Fault Tolerance

Open Telecom Platform

Supervisor

Application

GenServer

GenEvent

OOP

Objects

Elixir

Processes

Production-ready?

CoAP

CoAP - Codes

CoAP

parser

C

Hands on!

Goal:

Build a

Temperature Sensor

service that:

#1

Create a new project

#2

"Can read the service description"

#3

http
server

"Add 'register' functionality "