コンテキストでの型の操作 Monad型クラス ApplicativeとFunctorの制限 2つのMap.lookupを組み合わせる pure関数

入門Haskellプログラミング
 楽天ブックス
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開発環境

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 30(Monad型クラス)、30.1(ApplicativeとFunctorの制限)、2つのMap.lookupを組み合わせる、クイックチェック 30-1の解答を求めてみる。

得られる結果がMaybe PlayerCredits型ではなく、Maybe (Maybe PlayerCredits)型になってしまう。

実際に確認。

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

ghc-options:
- -Wall
- -Wcompat
- -Widentities
- -Wincomplete-record-updates
- -Wincomplete-uni-patterns
- -Wmissing-export-lists
- -Wmissing-home-modules
- -Wpartial-fields
- -Wredundant-constraints

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 (main) where

import Lib
  ( creditsFromIdStrange,
  )

main :: IO ()
main = do
  mapM_ (print . creditsFromIdStrange) [0 .. 4]

lesson/src/Lib.hs

module Lib
  ( creditsFromIdStrange,
  )
where

import qualified Data.Map as Map

type UserName = String

type GamerId = Int

type PalyerCredits = Int

userNameDB :: Map.Map GamerId String
userNameDB =
  Map.fromList
    [ (1, "name1"),
      (2, "name2")
    ]

creditsDB :: Map.Map String PalyerCredits
creditsDB =
  Map.fromList
    [ ("name1", 2000),
      ("name2", 15000)
    ]

lookupUserName :: GamerId -> Maybe UserName
lookupUserName gamerId = Map.lookup gamerId userNameDB

lookupCredits :: UserName -> Maybe PalyerCredits
lookupCredits username = Map.lookup username creditsDB

creditsFromIdStrange :: GamerId -> Maybe (Maybe PalyerCredits)
creditsFromIdStrange gamerId =
  pure lookupCredits
    <*> lookupUserName gamerId

入出力結果(Terminal, Zsh)

% stack exec lesson-exe
Nothing
Just (Just 2000)
Just (Just 15000)
Nothing
Nothing
%