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型によるプログラミング 直積型と直和型 ブックストアプログラムを作成する

入門Haskellプログラミング (Will Kurt(著)、株式会社クイープ(監修、翻訳)、翔泳社)のUNIT3(型によるプログラミング)、LESSON 16(直積型と直和型)、16.2(ブックストアプログラムを作成する)、クイックチェック 16-3の解答を求めてみる。

コード

lesson/app/Main.hs

module Main where

import Lib

main :: IO ()
main = do
  mapM_ print creators

lesson/src/Lib.hs

module Lib
  ( creators,
  )
where

storeItems :: [StoreItem]
storeItems =
  [ BookItem $ Book $ AuthorCreator $ Author $ Name "firstName" "lastName",
    RecordItem $
      VinylRecord $
        ArtistCreator $
          Person $
            NameWithMiddle "firstName" "middleName" "lastName",
    ToyItem $ CollectibleToy "toy"
  ]

creators :: [String]
creators = map madeBy storeItems

type FirstName = String

type LastName = String

type MiddleName = String

data Name
  = Name FirstName LastName
  | NameWithMiddle FirstName MiddleName LastName
  | TwoInitialsWithLast Char Char LastName
  deriving (Show)

data Author = Author Name deriving (Show)

data Artist = Person Name | Band String deriving (Show)

data Creator
  = AuthorCreator Author
  | ArtistCreator Artist
  deriving (Show)

data Book = Book
  { author :: Creator
  }

data VinylRecord = VinylRecord
  { artist :: Creator
  }

data CollectibleToy = CollectibleToy
  { name :: String
  }

data StoreItem
  = BookItem Book
  | RecordItem VinylRecord
  | ToyItem CollectibleToy

madeBy :: StoreItem -> String
madeBy (BookItem book) = show $ author book
madeBy (RecordItem vinylRecord) = show $ artist vinylRecord
madeBy _ = "unknown"

入出力結果(Terminal, Zsh)

% stack exec lesson-exe
"AuthorCreator (Author (Name \"firstName\" \"lastName\"))"
"ArtistCreator (Person (NameWithMiddle \"firstName\" \"middleName\" \"lastName\"))"
"unknown"
%