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コンテキストでの型の操作 Functor型クラス 例:Maybeでの計算

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)、27.1(例:Maybeでの計算)、クイックチェック 27-1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

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 Lib (reverseMaybe)

main :: IO ()
main = do
  mapM_
    (\s -> print [s, reverseMaybe s])
    [ Just "abcde",
      Nothing
    ]

lesson/src/Lib.hs

module Lib
  ( reverseMaybe,
  )
where

reverseMaybe :: Maybe String -> Maybe String
reverseMaybe Nothing = Nothing
reverseMaybe (Just val) = Just $ reverse val

入出力結果(Terminal, Zsh)

% stack exec lesson-exe
[Just "abcde",Just "edcba"]
[Nothing,Nothing]
%