Types in Ruby

Before starting

• Quick introduction
• Early development
• Comparison with other languages
• Very complicated and vast world

Why types are cool?

• Better IDE integration
• Give information to other developers
• Documentation for the code
• Can find bugs during development
• etc.

👉 TypeScript in 5 minutes

Why types are cool?

👉 Try it!

Typesception

Different type of types.

• Static & Dynamic typing
• Nominal & Structural typing
• Gradual typing

Static typing

  
// src/main/java/io/core/person/Person.java

public class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }
}

public class Main {
    public static void main(String[] args) {
        Person person = new Person("Alice", 30);
    }
}
  

Dynamic typing

  
# person.rb

class Person
  def initialize (name, age)
    @name = name
    @age = age
  end
end

person = new Person("Alice", 30)
  

Nominal typing

  
class Dog {
  public function speak() {
    return "whoaf";
  }
}

class Cat {
  ...
}

function speak(Dog $animal): void {
  echo $animal->speak();
}

speak(new Dog); ✅ //=> "whoaf"
speak(new Cat); ❌ Uncaught TypeError: speak(): Argument #1 ($animal) must be of type Dog, Cat given.
  

Structural typing

  
class Dog {
  speak () {
    return 'whoaf'
  }
}

const Cat = {
  speak () {
    return 'miaou'
  }
}

function speak (animal: Dog) {
  console.log(animal.speak())
}

speak(new Dog) ✅ //=> "whoaf"
speak(Cat) ✅ //=> "miaou"
  

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Gradual typing

• Gradually add static typing to dynamic languages
• Types are checked by linters
• Types are reported at runtime
• Nominal or Structural

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What about Ruby? 💎

"If it walks like a duck, and it quacks like a duck, then it must be a duck"

What about Ruby? 💎

Duck typing is about defining objects by what they do instead of who they are.

Duck typing in real life 🦆

  
class Trip
  def prepare(preparers)
    preparers.each do |preparer|
      case preparer
      when Mechanic then preparer.prepare_bicycles(bicycles)
      when TripCoordinator then preparer.buy_food(customers)
      when Driver
        preparer.gas_up(vehicle)
        preparer.fill_water_tank(vehicle)
      end
    end
  end
end

class Mechanic
  def prepare_bicycles(bicycles)
    #...
  end
end

class TripCoordinator
  def buy_food(customers)
    # ...
  end
end

class Driver
  def gas_up(vehicle)
    #...
  end

  def fill_water_tank(vehicle)
    #...
  end
end
  

Duck typing in real life 🦆

  
class Trip
  def prepare(preparers)
    preparers.each do |preparer|
      preparer.prepare_trip(self)
    end
  end
end

# when every preparer is a Duck that responds to ‘prepare_trip’
class Mechanic
  def prepare_trip(trip)
    prepare_bicycles(trip.bicycles)
  end
end

class TripCoordinator
  def prepare_trip(trip)
    buy_food(trip.customers)
  end
end

class Driver
  def prepare_trip(trip)
    gas_up(trip.vehicle)

    fill_water_tank(trip.vehicle)
  end
end
  

Duck typing in real life 🦆

Enforce it with modules

  
module Preparer
  def prepare_trip(trip)
    raise "Not implemented"
  end
end

class Trip
  def prepare(preparers)
    preparers.each do |preparer|
      preparer.prepare_trip(self)
    end
  end
end

class Mechanic
  include Preparer
end

class TripCoordinator
  include Preparer
end

class Driver
  include Preparer
end
  

Duck typing in real life 🦆

  
def import_state_badge(import)
  variant = {
    enqueued: :secondary,
    processing: :info,
    error: :danger,
    completed: :success
  }[import.state.to_sym]

  badge_tag(value: import.state.titleize, variant: variant)
end

<%= import_state_badge(@import) %>
<%= import_state_badge(@budget) %>
<%= import_state_badge("completed") %>
  

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The problem with Duck Typing 🦆

• No autocompletion
• Types are checked at runtime
• Hard to find occurrences
• No hint of what you can do or not

The problem with Duck Typing 🦆

Solution in Ruby

RBS in action

  
# working_day_calculator.rb

require 'date'

class WorkingDayCalculator
  attr_accessor :period_start, :period_end
  attr_reader :days_off

  def initialize
    @days_off = []
  end

  def add_day_off(date)
    @days_off << date
  end

  def calculate_working_days
    ((period_start..period_end).to_a - days_off).length
  end
end
  

RBS in action

  
# working_day_calculator.rbs

class WorkingDayCalculator
  attr_accessor period_start: Date
  attr_accessor period_end: Date
  attr_reader days_off: Array[Date]
  def initialize: -> void
  def add_day_off: (Date date) -> Array[Date]
  def calculate_working_days: -> untyped
end
  

RBS in action

  
# Type checking files:

....................................................................................F

working_day_calculator_test.rb:13:17: [error] Cannot pass a value of type `::String` as an argument of type `::Date`
│   ::String <: ::Date
│     ::Object <: ::Date
│       ::BasicObject <: ::Date
│
│ Diagnostic ID: Ruby::ArgumentTypeMismatch
│
└ calc.add_day_off('BOOM')
                   ~~~~~~~~~~

Detected 1 problem from 1 file
  

RBS inline

https://github.com/soutaro/rbs-inline

  
# rbs_inline: enabled

class Person
  attr_reader :name #: String

  attr_reader :addresses #: Array[String]

  # @rbs name: String
  # @rbs addresses: Array[String]
  # @rbs return: void
  def initialize(name:, addresses:)
    @name = name
    @addresses = addresses
  end

  def to_s #: String
    "Person(name = #{name}, addresses = #{addresses.join(", ")})"
  end

  # @rbs &block: (String) -> void
  def each_address(&block) #:: void
    addresses.each(&block)
  end
end
  

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Conclusion

• Still early stage of development
• But the pace is not great...
• It's important for Ruby adoption
• RBS roadmap
• Far from being production ready
• Should copy TypeScript DX

Thanks ! 🙏

• https://types-in-ruby.guillaumebriday.fr