All I Learned While Working on a Scala OSS Project for Over Six Years

https://speakerdeck.com/seratch/all-i-learned-while-working-on-a-scala-oss-project-for-over-six-years-number-scalamatsuri

https://youtu.be/y7BxvT-Jm6w

Kazuhiro Sera (@seratch)

Impressions

• Scala has come a long way :)

• Things we take for granted now:
  • IDE integration
  • Named and default arguments
  • Minor ver. binary compatibility
  • String interpolation
  • JavaConver[ters|sions]
  • etc.

Key Takeaways

• Consider use of third-party libraries:
  • Minimise Scala third-party library dependencies, or
  • Use Java libraries (and deal with nulls)

• Use CI and test cross-build artifacts

• Support deprecated APIs (Scala or third party libs) via version-specific modules

https://github.com/lightbend/migration-manager

• Use "MiMa": Lightbend's Migration Manager

A Pragmatic Introduction to Category Theory

https://speakerdeck.com/danielasfregola/scalamatsuri-2018-a-pragmatic-introduction-to-category-theory

https://youtu.be/9Ab5JhCQAF0

Daniela Sfregola (@DanielaSfregola)

Impressions

• "Monad is a Monoid in the category of endofunctors"

• Attendees can follow along with "fill in the blanks" examples:

• Great, guided explanations of:
  • Monoid
  • Functor
  • Applicative
  • Monad

https://github.com/DanielaSfregola/tutorial-cat

Key Takeaways

• A fundamental part of Functional Programming (with Type System)

• Category theory, simply the laws of composition and abstraction:
  • Maths of mathematics, or
  • Arrow theory, or
  • Transformation law,
  • etc.

• Category rules:
  • Identity
  • Composition
  • Associativity

• Category methods (in Scala):
  • `pure` - lifting to a context
  • `flatMap` - fusing context
  • `map` - transform in context

Further Resources

• Category Theory for Programmers:
  https://bartoszmilewski.com/2014/10/28/category-theory-for-programmers-the-preface/

• Herding Cats:
  http://eed3si9n.com/herding-cats/
  

• Functional Programming in Kotlin with Arrow:
  https://youtu.be/FcaaTJhCEcw
  

Monad Transformers for the Working Programmer

https://speakerdeck.com/gabro/monad-transformers-down-to-earth-4

https://youtu.be/t9WvQuqJVU0

Gabriele Petronella (@gabro27)

Impressions

• A practical example of Category Theory

• Procedural shell, functional core: leveraging `for` comprehensions syntactic sugar

• Great example, retrieving a User and their Address in various shapes:
  • From:
    Future[Option[User]]
    Future[Option[Address]]
  • To:
    Monad[FutOpt[User]]
    Monad[FutOpt[Address]]

Key Takeaways

• Category Theory is a practical solution to everyday problems

• Three functional composition styles and libraries of interest:
  • Free Monad
  • Tagless final
  • Extensible Effects (Eff)

• Be aware of tradeoffs:
  • Stacking two Monads gets hairy really quickly
  • Keep transformers as internal implementations
  • Wrapping and unwrapping is not cheap
  • Use them as local optimisations

Purely Functional Play Framework Application

https://www.slideshare.net/AoiroAoino/purelyfunctionalplayframeworkapplication

https://youtu.be/LtnT8SPKAqU

Naoki Aoyama (@AoiroAoino)

Impressions

• Google Guice has been used in Play Framework, since forever ;)

• Application of Free Monad / Tagless final style in Play Framework

• Move away from runtime DI, and make it a compiler error instead of runtime error

• Separate program and execution:
  • ReaderMonad
  • WriterMonad

• Good example of persisting User record(s), and abstracting away implicits to the interpreter's context (e.g. Future or Task)

Key Takeaways

• Better support in latest Play Framework (2.6)

• To start with, use an opinionated library?

• Need more examples, in practice:
  • Stacking Monads
  • Composing concrete types
  • Use natural transformation and mapK

http://frees.io/docs/integrations/play/

Some other talks?

Extensible Effects in Dotty

Sanshiro Yoshida (@halcat0x15a)

https://www.slideshare.net/SanshiroYoshida/extensible-effects-in-dotty

https://youtu.be/QeW8Cka28vs

Functional Performance

Martin Thompson (@mjpt777)

https://youtu.be/xUy7xZjA04U