📜Recap

Union types: Option and Result

• What they are used for:

  • Model absence of value and business errors

  • Partial operations made total tryXxx

    • Smart constructor tryCreate

• How to use them:

  • Chaining: map, bind, filter → ROP

  • Pattern matching

• Their benefits:

  • null free, Exception free → no guard clauses Cluttering the code

  • Makes business logic and happy path more readable

Computation expression (CE)

• Syntactic sugar: inner syntax standard or "banged" (let!)

• Separation of Concerns: business logic vs "machinery"

• Compiler translates to builder calls

  • Object storing a state

  • Builds an output value of a specific type

• Can be nested but not easy to combine!

• Underlying theoretical concepts

  • Monoid → seq (of composable elements and with a "zero ")

  • Monad → async, option, result

  • Applicative → validation/Result<'T, 'Err list>

• Libraries: FSharpPlus, FsToolkit, Expecto, Farmer, Saturn, Bolero...

🔗 Additional ressources

• Compositional IT (Isaac Abraham)

  • Writing more succinct C# – in F#! (Part 2) • 2020
• F# for Fun and Profit (Scott Wlaschin)

  • The Option type • 2012
  • Making illegal states unrepresentable • 2013
  • The "Map and Bind and Apply, Oh my!" series • 2015
  • The "Computation Expressions" series • 2013
• Extending F# through Computation Expressions

  • 📹 Video
  • 📜 Article
• Computation Expressions Workshop
• Applicatives IRL by Jeremie Chassaing