Haskell - コンテキストでの型の操作 - Monad型クラス - ApplicativeとFunctorの制限

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)、クイックチェック 30-1の解答を求めてみる。

コード

sample1.hs

import qualified Data.Map as Map
main :: IO ()
main = do
    print $ creditsFromIdStrange 1
    print $ creditsFromIdStrange 10
    
type UserName = String
type GamerId = Int
type PlayerCredits = Int

userNameDB :: Map.Map GamerId UserName
userNameDB = Map.fromList [(1, "nYarLathoTep"),
    (2, "KINGinYELLOW"),
    (3, "dagon1997"),
    (4, "rcarter1919"),
    (5, "xCTHULHUx"),
    (6, "yogSOThoth")]

creditsDB :: Map.Map UserName PlayerCredits
creditsDB = Map.fromList [("nYarLathoTep", 2000),
    ("KINGinYELLOW", 15000),
    ("dagon1997",300),
    ("rcarter1919",12),
    ("xCTHULHUx",50000),
    ("yogSOThoth",150000)]
lookupUserName :: GamerId -> Maybe UserName
lookupUserName id = Map.lookup id userNameDB

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

-- Maybeが二重になる
creditsFromIdStrange :: GamerId -> Maybe (Maybe PlayerCredits)
creditsFromIdStrange id = pure lookupCredits <*> lookupUserName id

入出力結果(Terminal, Zsh)

% runghc sample1.hs 
Just (Just 2000)
Nothing
%