影片筆記: Domain Modeling Made Functional by Scott Wlaschin

https://www.youtube.com/watch?v=2JB1_e5wZmU

https://www.slideshare.net/ScottWlaschin/domain-modeling-made-functional-kandddinsky-2019

講者 Scott Wlaschin 是 Domain Modeling Made Functional 的作者

前言

之後主要從下面這個例子展開，展示 type 的重要和在設計中的美妙之處

type Contact = {

  FirstName: string
  MiddleInitial: string
  LastName: string

  EmailAddress: string
  IsEmailVerified: bool
}

Functional programming is really good for… Boring Line Of Business Applications (BLOBAs)

Part.1 The importance of design

下面是一個 F# 的程式碼，即使沒有學過 F# 甚至沒有學過程式語言，還是可以很清楚地猜到這應該是個撲克牌的遊戲

module CardGame =
  type Suit = Club | Diamond | Spade | Heart

  type Rank = Two | Three | Four | Five | Six | Seven | Eight
              | Nine | Ten | Jack | Queen | King

  type Card = Suit * Rank

  type Hand = Card list
  type Deck = Card list

  type Player = { Name:string; Hand:Hand }
  type Game = { Deck:Deck; Players: Player list }

  type Deal = Deck –› (Deck * Card)

  type PickupCard = (Hand * Card) –› Hand

在這裡，動作被 model 成 function

type Deal = Deck –› (Deck * Card)

在 Deal 這個 function 中，input 是 Deck，而 output 是一個新的 Deck 和一張 Card

type PickupCard = (Hand * Card) –› Hand

而在 PickupCard 這個 function 中，input 是 Hand 和 Card，而 output 是新的 Hand

回到一開始 Contact 的例子，

這些欄位都一定要有嗎？ 不一定. 例如不一定有 MiddleName

FirstName 的 type 是 String，但他真的可以是任意 String 嗎？ 不. 他應該要限制長度、哪些字元等等

Part.2 Understanding FP type systems

FP principle: types are not classes

type 是 sets

FP principle: composition everywhere

• compose with AND

s ss

• compose with OR

s ss

FP principle:

Part.3 Domain modeling with composable types ?

Modeling optional values

Modeling simple values and constraint values

…

Replacing flags with choices

Part.4 Encoding business rules with types

Summary

Use code to represent the shared mental model and ubiquitous language

Designs will evolve. Embrace change.

• Refactor towards deeper insight
• Static types give you confidence to make changes

Use the power of a composable type system

• Choices rather than inheritance
• Options instead of null
• Wrappers for constrained types
• Make illegal states unrepresentable