talk.md

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Going Functional with Algebraic Effects

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👋

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Me

Nikita Shilnikov

• I write code
• I write code in Ruby

 ` `` `` ` 

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Algebraic effects?

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#hype

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Algebraic effects are a new burrito

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Already in production

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React is wired with algebraic effects

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Functional programming?

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[.code: text-scale(1.6)]

f = -> x { x + 5 }

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[.code: text-scale(1.6)]

Integer => Integer

f = -> x { x + 5 }

---

[.code: text-scale(1.6)]

f = -> x { puts(x); x + 5 }

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Launching rockets problem

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[.code: text-scale(1.6)]

doSomethingNice :: Integer -> IO ()

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[.code: text-scale(1.6)]

Integer => Integer, ?

f = -> x { puts(x); x + 5 }

---

[.code: text-scale(1.6)]

Integer => Puts[Integer] : Integer

f = -> x { puts(x); x + 5 }

---

[.code: text-scale(1.6)] [.code-highlight: 1-2]

Integer => Puts[Integer] : Integer
           ^^^^^^^^^^^^^
f = -> x { puts(x); x + 5 }

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Puts[Integer] is an effect

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[.code: text-scale(1.6)]

g = -> x { x + get(:y) }

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[.code: text-scale(1.6)]

Integer => Get[Symbol => Integer] : Integer

g = -> x { x + get(:y) }

---

[.code: text-scale(1.6)] [.code-highlight: 1-2]

Integer => Get[Symbol => Integer] : Integer
           ^^^^^^^^^^^^^^^^^^^^^^
g = -> x { x + get(:y) }

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Effects in type signatures
reveal code's intentions

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Effects
are not
side effects

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[.code: text-scale(1.3)] [.text: alignment(left)]

f = -> x { puts(x); x + 5 }

Side effects: Integer => Integer

Effects: Integer => Puts[Integer] : Integer

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To run effectful code
you'll need a handler

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[.code: text-scale(1.5)]

f = -> x { puts(x); x + 5 }

with_puts { f.(10) }

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[.code: text-scale(1.5)] [.code-highlight: 2-3]

f = -> x { puts(x); x + 5 }

with_puts { f.(10) }
^^^^^^^^^

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Every effect must have
a handler

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[.code: text-scale(1.5)]

f = -> x { puts(x); x + 5 }

f.(10) # => Error!

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The World

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[.code: text-scale(1.6)]

def greet
  print "Hello"
end

def main
  handle_print { greet }
end

---

[.code: text-scale(1.8)]

main
handle_print
greet
print

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[.code: text-scale(1.8)]

               World
main         # |
handle_print # |
greet        # |
print        # ↓

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[.code: text-scale(1.8)]

               World
main         # |
handle_print # | *
greet        # | ↑
print        # ↓ |

---

[.code: text-scale(1.8)]

               World
main         # |
handle_print # | * |
greet        # | ↑ |
print        # ↓ | ↓

---

[.code: text-scale(1.8)]

               World
main         # | ↑ |
             # | | |
greet        # | | |
print        # ↓ | ↓

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World is the Handler

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It's a side effect when
it's handled by the World

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Examples 🌈

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Passing pseudo-global values around

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[.code: text-scale(1.6)]

class SetLocaleMiddleware
  def call(env)
    locale = detect_locale(env)
    with_locale(locale) { @app.(env) }
  end
end

---

Testing features

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[.code: text-scale(1.5)]

Testing features

class RenderView
  def call(values)
    if feature?
      render_with_feature(values)
    else
      render_without_feature(values)
    end
  end
end

---

[.code: text-scale(1.5)] [.code-highlight: 1-4,9]

Testing features

def call(env)
  feature_response, no_feature_response = with_feature do
    @app.(env)
  end

  if feature_response != no_feature_response
    # ...
  end
end

---

Testing features

def call(env)
  feature_response, no_feature_response = with_feature do
    @app.(env)
  end

  if feature_response != no_feature_response
    # ...
  end
end

---

dry-effects

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Controlling time

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Accessing current time

class CreatePost
  include Dry::Effects.CurrentTime

  def call(values)
    publish_at = values[:publish_at] || current_time
    # ...
  end
end

---

[.code-highlight: 1,4,6-7]

Providing current time

class WithCurrentTime
  include Dry::Effects::Handler.CurrentTime

  def call(env)
    with_current_time { @app.(env) }
  end
end

---

[.code-highlight: 2,5]

Providing time

class WithCurrentTime
  include Dry::Effects::Handler.CurrentTime

  def call(env)
    with_current_time { @app.(env) }
  end
end

---

Testing

include Dry::Effects::Handler.CurrentTime

example do
  with_current_time { ... }
end

---

Testing

RSpec.configure do |c|
  c.include Dry::Effects::Handler.CurrentTime

  now = Time.now
  c.around do |ex|
    with_current_time(proc { now }, &ex)
  end
end

---

Testing

example do
  next_day = Time.now + 86_400

  with_current_time(proc { next_day }) { ... }
end

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Dependency injection

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Dependency injection

class CreatePost
  include Dry::Effects.Resolve(:post_repo)

  def call(values)
    if valid?(values)
      post_repo.create(values)
    else
      ...
    end
  end
end

---

[.code-highlight: 2]

Dependency injection

class CreatePost
  include Dry::Effects.Resolve(:post_repo)

  def call(values)
    if valid?(values)
      post_repo.create(values)
    else
      ...
    end
  end
end

---

[.code-highlight: 2,6]

Dependency injection

class CreatePost
  include Dry::Effects.Resolve(:post_repo)

  def call(values)
    if valid?(values)
      post_repo.create(values)
    else
      ...
    end
  end
end

---

Making a container

AppContainer = {
  post_repo: PostRepo.new,
  ...
}

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Providing dependencies

class ProvideApplication
  include Dry::Effects::Handler.Resolve(AppContainer)

  def call(env)
    provide { @app.(env) }
  end
end

---

[.code-highlight: 2,5]

Providing dependencies

class ProvideApplication
  include Dry::Effects::Handler.Resolve(AppContainer)

  def call(env)
    provide { @app.(env) }
  end
end

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Testing

include Dry::Effects::Handler.Resolve

example do
  post_repo = double(:post_repo)
  provide(post_repo: post_repo) do
    # ...
  end
end

---

[.code: text-scale(1.5)]

Tracing

AppContainer = AppContainer.to_h do |key, value|
  [key, Wrapper.new(value)]
end

---

[.code: text-scale(1.5)]

Tracing

AppContainer = AppContainer.to_h do |key, value|
  [key, Wrapper.new(value)]
end

---

Batteries included

Dry::Effects.load_extensions(:system)

class App < Dry::Effects::System::Container
  Import = injector(...)
end

---

[.code-highlight: 7-9]

Frozen application

Dry::Effects.load_extensions(:system)

class App < Dry::Effects::System::Container
  Import = injector(...)
end

# boot.rb

App.finalize!

---

[.code: text-scale(1.5)]

class CreateUser
  def initialize
  end

  def call
  end
end

---

State

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State

class Add
  include Dry::Effects.State(:result)

  def call(b)
    self.result += b
    nil
  end
end

---

State

class Mult
  include Dry::Effects.State(:result)

  def call(b)
    self.result *= b
    nil
  end
end

---

State

class Calc
  include Dry::Effects::Handler.State(:result)

  def add
    Add.new
  end

  def mult
    Mult.new
  end
end

---

State

def call(x, y, z)
  with_result(x) do
    add.(y)
    mult.(z)
    "🍏"
  end
end

---

State

calc = Calc.new
calc.(5, 6, 7)

---

State

calc = Calc.new
calc.(5, 6, 7) # => (5 + 6) * 7 == 77

---

State

calc = Calc.new
calc.(5, 6, 7)
# => [77, "🍏"]

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All effects are composable*

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Composition

class Program
  include Dry::Effects.Cmp(:feature)
  include Dry::Effects.State(:counter)

  def call
    if feature?
      self.counter += 2
      "bye"
    else
      self.counter += 1
      "hi!"
    end
  end
end

---

program = Program.new

Dry::Effects[:state, :counter].(10) do
  Dry::Effects[:cmp, :feature].() do
    program.()
  end
end
# => [13, ["hi!", "bye!"]]

---

program = Program.new

Dry::Effects[:cmp, :feature].() do
  Dry::Effects[:state, :counter].(10) do
    program.()
  end
end
# => [[11, "hi!"], [12, "bye!"]]

---

More examples

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Timeout

class MakeRequest
  include Dry::Monads[:try]
  include Dry::Effects.Timeout(:http)

  def call(url)
    Try() { HTTParty.get(url, timeout: timeout) }
  end
end

---

Timeout

class TimeoutMiddleware
  include Dry::Effects::Handler.Timeout(:http)

  def call(env)
    with_timeout(5.0) { @app.(env) }
  end
end

---

Parallel

class PullData
  include Dry::Effects.Parallel

  def call(urls)
    join(urls.map { |url| par { make_request.(url) } })
  end
end

---

[.code-highlight: 5-6]

Parallel

class PullData
  include Dry::Effects.Parallel

  def call(urls)
    join(urls.map { |url| par { make_request.(url) } })
    ^^^^                  ^^^
end

---

Parallel

class ParallelMiddleware
  include Dry::Effects::Handler.Parallel

  def call(env)
    with_parallel.() { @app.(env) }
  end
end

---

dry-effects is a practical-oriented implementation

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v0.1

Cache, Cmp, CurrentTime, Defer, Env, Implicit, Interrupt, Lock, Parallel, Random, Reader, Resolve, Retry, State, Timeout, Timestamp

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Effects almost don't affect existing code

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Just like monads, effects are language agnostic

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Many shades of algebraic effects

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Level 0

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React

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React.useState

Dry::Effects.CurrentTime

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Level 1

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dry-effects

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Dry::Effects.Retry

Dry::Effects.Parallel

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Level 2

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Fibers

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Async/await

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Async/await

server.rb     # scheduler
...           #  ↑
user_repo.rb  # find_user

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Async/await

server.rb     # scheduler
...           #  ↑  ↓
user_repo.rb  # find_user

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Level 3

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Multi-shot continuations

server.rb     # scheduler
...           #  ↑  ↓ ↓ ↓
user_repo.rb  # find_user

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Multi-shot continuations allow
backtracking, parsers, etc.

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callcc / Fiber#dup

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Level 4

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Typed effects

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Algebraic effects are coming

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It works

Trust me!

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Pros

• New abilities
• Easy to use
• Already works (React!)
• Easy to test
• Traceable effects

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Cons

• Unfamiliar
• Can be overused
• Can be abused
• Require glue code with threading

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Next steps for dry-effects

• Add async/await
• Polishing APIs
• More integrations with existing gems
• More docs and examples
• Multi-shot continuations?

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It's not all

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Learn more

• github.com/topics/algebraic-effects
• github.com/yallop/effects-bibliography

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Thank you

• twitter.com/NikitaShilnikov
• dry-rb.org/gems/dry-effects
• dry-rb.org/gems/dry-system
• github.com/dry-rb/dry-effects
• t.me/flash_gordon

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Questions?