Haskell - コンテキストでの型の操作 - Functor型クラス - fmap関数, <$>演算子, Maybe型

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 27(Functor型クラス)、27.5(練習問題)Q27-3の解答を求めてみる。

コード

sample.hs

import Data.Map qualified as Map
import System.Environment (getArgs)

main :: IO ()
main = do
  args <- getArgs
  let n = (read $ head args) :: Int
  let part = Map.lookup n partsDB
  print $ cost <$> part

data RobotParts = RobotParts
  { name :: String,
    cost :: Double
  }
  deriving (Show)

leftArm :: RobotParts
leftArm =
  RobotParts
    { name = "left arm",
      cost = 1000.00
    }

rightArm :: RobotParts
rightArm =
  RobotParts
    { name = "right arm",
      cost = 1025.00
    }

robotHead :: RobotParts
robotHead =
  RobotParts
    { name = "robot head",
      cost = 5092.25
    }

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

入出力結果(Terminal, Zsh)

% runghc sample.hs 1
Just 1000.0
% runghc sample.hs 2
Just 1025.0
% runghc sample.hs 3
Just 5092.25
% runghc sample.hs 4
Nothing
% runghc sample.hs a
sample.hs: Prelude.read: no parse
%