Well Typed Elixir - Speaker Deck

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Well Typed Elixir Chris Keathley / @ChrisKeathley / keathley.io

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Lets talk about:

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Lets talk about: • Why should we care about types?

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Lets talk about: • Why should we care about types? • Types 101 & Dialyzer

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Lets talk about: • Why should we care about types? • Types 101 & Dialyzer • Creating our own types

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Lets talk about: • Why should we care about types? • Types 101 & Dialyzer • Creating our own types • Type Composition

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Lets talk about: • Why should we care about types? • Types 101 & Dialyzer • Creating our own types • Type Composition • Putting it all together

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Why should we care about types?

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Monads

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Lets not talk about monads

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But here’s the gist if you’re interested https://git.io/v2jzJ

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“This is Erlang. Just Let it Crash™”

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Types are not a replacement for OTP

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Mistake Proofing

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Complexity

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Types 101

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defmodule ElixirTypes do def run do calculate_balance(50, 30) end def calculate_balance(a, b), do: a - b end

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defmodule ElixirTypes do def run do calculate_balance(50, :some_atom) end def calculate_balance(a, b), do: a - b end

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defmodule ElixirTypes do def run do calculate_balance(50, :some_atom) end def calculate_balance(a, b), do: a - b end LOL

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Dialyzer

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github.com/jeremyjh/dialyxir github.com/fishcakez/dialyze

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$ mix dialyzer.plt

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defmodule ElixirTypes do def run do balance = 50 transaction = pay(:andra, 20) calculate_balance(balance, money(transaction)) end def pay(name, amount), do: {name, amount} def money({amount, _name}), do: amount def calculate_balance(a, b), do: a - b end

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Dialyzer

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ಠ_ಠ

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Why is this not an error?

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A bit of history

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def send(pg, mess) when is_atom(pg) do :global.send(pg, {:send, self(), mess}) end def send(pg, mess) when is_pid(pg) do send(pg, self(), mess) end

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Success Types

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def and(true, true), do: true def and(_, false), do: false def and(false, _), do: false

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and :: bool() -> bool() -> bool()

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and :: any() -> any() -> bool()

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A Type is a Proposition

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and(true, true) # => true and(“foo", false) # => false and(false, 3) # => false

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Dialyzer is optimistic

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If Dialyzer complains there’s almost certainly a problem

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We need to help dialyzer out

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@typespecs

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@type balance :: integer() @type transaction :: {atom(), integer()}

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defmodule ElixirTypes do @type balance :: integer() @type transaction :: {atom(), integer()} def run do balance = 50 transaction = pay(:andra, 20) calculate_balance(balance, money(transaction)) end def pay(name, amount), do: {name, amount} def money({amount, _name}), do: amount def calculate_balance(a, b), do: a - b end

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defmodule ElixirTypes do @type balance :: integer() @type transaction :: {atom(), integer()} def run do balance = 50 transaction = pay(:andra, 20) calculate_balance(balance, money(transaction)) end @spec pay(atom, integer) :: transaction def pay(name, amount), do: {name, amount} @spec money(transaction) :: integer def money({amount, _name}), do: amount @spec calculate_balance(integer, integer) :: balance def calculate_balance(a, b), do: a - b end

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(˽°□°҂˽Ɨ ˍʓˍ

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dialyzer: [ flags: ["-Woverspecs"] ]

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defmodule ElixirTypes do @type balance :: integer @type transaction :: {atom, integer} def run do balance = 50 transaction = pay(:andra, 20) calculate_balance(balance, money(transaction)) end @spec pay(atom, integer) :: transaction def pay(name, amount), do: {name, amount} @spec money(transaction) :: integer def money({amount, _name}), do: amount @spec calculate_balance(integer, integer) :: balance def calculate_balance(a, b), do: a - b end

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defmodule ElixirTypes do @type balance :: integer @type transaction :: {atom, integer} def run do balance = 50 transaction = pay(:andra, 20) calculate_balance(balance, money(transaction)) end @spec pay(atom, integer) :: transaction def pay(name, amount), do: {name, amount} @spec money(transaction) :: integer def money({amount, _name}) when is_integer(amount) do amount end @spec calculate_balance(integer, integer) :: balance def calculate_balance(a, b), do: a - b end

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defmodule ElixirTypes do @type balance :: integer @type transaction :: {atom, integer} def run do balance = 50 transaction = pay(:andra, 20) calculate_balance(balance, money(transaction)) end @spec pay(atom, integer) :: transaction def pay(name, amount), do: {name, amount} @spec money(transaction) :: integer def money({_name, amount}) when is_integer(amount) do amount end @spec calculate_balance(integer, integer) :: balance def calculate_balance(a, b), do: a - b end

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@type balance :: integer @type transaction :: {name, integer} @type name :: atom | String.t

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defmodule ElixirTypes do @type name :: atom | String.t @type transaction :: {name, integer} @type ledger(type) :: {:ledger, list(type), list(type)} @spec new :: ledger(transaction) def new() do {:ledger, [pay(:andra, 20)], [pay(:chris, 10)]} end @spec pay(atom, integer) :: transaction def pay(name, amount), do: {name, amount} end

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Creating our own data types

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Structs

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defmodule Queue do defstruct inbox: [], outbox: [] end

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defmodule Queue do defstruct inbox: [], outbox: [] @type t :: %__MODULE__{ inbox: list(), outbox: list() } end

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defmodule Queue do defstruct inbox: [], outbox: [] @type t :: %__MODULE__{ inbox: list(), outbox: list() } @spec push(Queue.t, any()) :: Queue.t @spec pop(Queue.t) :: {any(), Queue.t} @spec new :: Queue.t end

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def run do Queue.new |> Queue.push(:foo) |> Queue.push(:bar) |> Queue.pop end

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defmodule Queue do defstruct inbox: [], outbox: [] @spec size(Queue.t) :: non_neg_integer def size(%Queue{inbox: i, outbox: o}) do Enum.count(i) + Enum.count(o) end end

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defmodule Queue do defstruct inbox: [], outbox: [] @spec size(Queue.t) :: non_neg_integer def size(%Queue{inbox: i, outbox: o}) do Enum.count(i) + Enum.count(o) end end defmodule Tree do defstruct key: nil, left: nil, right: nil @spec size(Tree.t) :: non_neg_integer def size(%Tree{left: left, right: right}) do size(left) + size(right) + 1 end def size(nil), do: 0 end

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defprotocol Sizing do @spec size(any()) :: integer def size(structure) end

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defimpl Sizing, for: Queue do def size(%Queue{inbox: i, outbox: o}) do Enum.count(i) + Enum.count(o) end end defimpl Sizing, for: Tree do def size(%Tree{left: left, right: right}) do size(left) + size(right) + 1 end def size(nil), do: 0 end

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def run do Queue.new |> Queue.push(:foo) |> Sizing.size # => 1 Tree.new({1, :foo}) |> Tree.insert({3, :bar}) |> Sizing.size # => 2 end

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Protocols are opt in polymorphism

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We gain more use from our common apis

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defprotocol Sizing do @spec empty?(any()) :: boolean def empty?(structure) @spec size(any()) :: non_neg_integer def size(structure) end

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defimpl Sizing, for: Tree do def empty?(t), do: size(t) > 0 def size(%Tree{left: left, right: right}) do size(left) + size(right) + 1 end def size(nil), do: 0 end defimpl Sizing, for: Queue do def empty?(q), do: size(q) > 0 def size(%Queue{inbox: i, outbox: o}) do Enum.count(i) + Enum.count(o) end end

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defmodule Presence do def empty?(thing), do: size(thing) == 0 def size(thing), do: Presence.Sizing.size(thing) end defprotocol Presence.Sizing do @spec size(any()) :: non_neg_integer def size(structure) end

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defimpl Presence.Sizing, for: Tree do def size(%Tree{left: left, right: right}) do size(left) + size(right) + 1 end def size(nil), do: 0 end defimpl Presence.Sizing, for: Queue do def size(%Queue{inbox: i, outbox: o}) do Enum.count(i) + Enum.count(o) end end

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defmodule Presence do def present?(thing), do: size(thing) > 0 def empty?(thing), do: size(thing) == 0 def size(thing), do: Presence.Sizing.size(thing) end defprotocol Presence.Sizing do @spec size(any()) :: non_neg_integer def size(structure) end

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def run do Queue.new |> Queue.push(:foo) |> Presence.size # => 1 Tree.new({1, :foo}) |> Tree.insert({3, :bar}) |> Presence.size # => 2 end

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Type composition

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f2 :: B() -> C() f1 :: A() -> B()

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B() -> C() A() -> B()

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A() -> B() -> C()

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A() -> (Class b) -> C()

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A() -> (Presence any()) -> C()

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Putting it together

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Email Slack Ticketing system Ticket processing Notification

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defmodule Alerts.Ticket.Email do defstruct from: nil, body: "", title: "" end defmodule Alerts.Ticket.SlackMessage do defstruct from: nil, message: "", channel: nil end

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defmodule Alerts.Adapters.SlackMessage do def receive(json) when is_binary(json) do json |> Poison.decode(as: %Slack{}) |> Ticket.process end end defmodule Alerts.EmailAdapter do def receive(email) when is_binary(email) do email |> Email.parse_from_email_string(email) |> Ticket.process end end

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defmodule Alerts.Ticket do @type requester :: atom | String.t @type message :: String.t @type severity :: 1..5 @type t :: %{ requester: requester, message: message, severity: severity } defstruct requester: nil, message: nil, severity: 1 def process(data) do ticket = to_ticket(data) TicketQueue.push(ticket) Alerts.Notifier.notify_on_call_staff(ticket) end @spec to_ticket(any()) :: t def to_ticket(data) do Alerts.Ticket.Fields.to_ticket(data) end end

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defmodule Alerts.Notifier do alias Alerts.Ticket @spec notify_on_call_staff(Ticket.t) :: Email.t def notify_on_call_staff(%Ticket{}) do # ... end end defmodule Alerts.TicketQueue do alias Alerts.Ticket @spec push(Ticket.t) :: TicketQueue.t def push(%Ticket{}) do # ... end end

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defprotocol Alerts.Ticket.Fields do @spec to_ticket(any()) :: Alerts.Ticket.t def to_ticket(thing) end

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defimpl Alerts.Ticket.Fields, for: Alerts.Ticket.Email do def to_ticket(email) do %Alerts.Ticket{ requester: email.from, message: email.body, severity: severity(email.title) } end def severity(title), do: # ... end defimpl Alerts.Ticket.Fields, for: Alerts.Ticket.SlackMessage do def to_ticket(msg) do %Alerts.Ticket{ requester: msg.from, message: msg.message, severity: severity(msg.message) } end def severity(message), do: #... end

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Future work

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Something that understands elixir a bit better

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Building the PLT sucks

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Better errors

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Resources • Success Types - http://user.it.uu.se/~kostis/Papers/succ_types.pdf • http://blog.plataformatec.com.br/2014/09/writing-assertive-code-with- elixir/ • http://learnyousomeerlang.com/dialyzer • http://learnyousomeerlang.com/types-or-lack-thereof

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Thanks Chris Keathley / @ChrisKeathley / keathley.io