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コンテキストでの型の操作 Functor型クラス Functorはいつもそばにいる リスト、fmap関数とmap関数、<$>演算子

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UNIT5(コンテキストでの型の操作)、LESSON 27(Functor型クラス)、27.3(Functorはいつもそばにいる)、RobotPartのリストをHTMLのリストに変換する、クイックチェック 27-3の解答を求めてみる。

lesson/package.yaml

name:                lesson
version:             0.1.0.0
github:              "githubuser/lesson"
license:             BSD3
author:              "Author name here"
maintainer:          "example@example.com"
copyright:           "2022 Author name here"

extra-source-files:
- README.md
- ChangeLog.md

# Metadata used when publishing your package
# synopsis:            Short description of your package
# category:            Web

# To avoid duplicated efforts in documentation and dealing with the
# complications of embedding Haddock markup inside cabal files, it is
# common to point users to the README.md file.
description:         Please see the README on GitHub at <https://github.com/githubuser/lesson#readme>

dependencies:
- base >= 4.7 && < 5
- containers

library:
  source-dirs: src

executables:
  lesson-exe:
    main:                Main.hs
    source-dirs:         app
    ghc-options:
    - -threaded
    - -rtsopts
    - -with-rtsopts=-N
    dependencies:
    - lesson

tests:
  lesson-test:
    main:                Spec.hs
    source-dirs:         test
    ghc-options:
    - -threaded
    - -rtsopts
    - -with-rtsopts=-N
    dependencies:
    - lesson

コード

lesson/app/Main.hs

module Main where

import qualified Data.Map as Map
import Lib

leftArm :: RobotPart
leftArm =
  RobotPart
    { name = "left arm"
    }

rightArm :: RobotPart
rightArm =
  RobotPart
    { name = "right arm"
    }

robotHead :: RobotPart
robotHead =
  RobotPart
    { name = "robot head"
    }

partsDB :: Map.Map Int RobotPart
partsDB =
  Map.fromList $
    zip
      [1, 2, 3]
      [leftArm, rightArm, robotHead]

allParts :: [RobotPart]
allParts = snd <$> Map.toList partsDB

main :: IO ()
main = do
  print partsDB
  print $ Map.toList partsDB
  print allParts

lesson/src/Lib.hs

module Lib where

data RobotPart = RobotPart
  { name :: String
  }
  deriving (Show)

入出力結果(Terminal, Zsh)

% stack exec lesson-exe
fromList [(1,RobotPart {name = "left arm"}),(2,RobotPart {name = "right arm"}),(3,RobotPart {name = "robot head"})]
[(1,RobotPart {name = "left arm"}),(2,RobotPart {name = "right arm"}),(3,RobotPart {name = "robot head"})]
[RobotPart {name = "left arm"},RobotPart {name = "right arm"},RobotPart {name = "robot head"}]
%