計算機科学のブログ

実践Haskell データベースの使用 データの作成:ユーザーの挿入とツールの貸し出し データベースに新しいユーザーを追加する

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UNIT7(実践Haskell)、LESSON41(Haskellでのデータベースの使用)、41.3(データの作成:ユーザーの挿入とツールの貸し出し)、データベースに新しいユーザーを追加する、クイックチェック 41-2の解答を求めてみる。

package.yaml

name:                db-lesson
version:             0.1.0.0
github:              "githubuser/db-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/db-lesson#readme>

dependencies:
- base >= 4.7 && < 5

library:
  source-dirs: src

executables:
  db-lesson-exe:
    main:                Main.hs
    source-dirs:         app
    ghc-options:
    - -threaded
    - -rtsopts
    - -with-rtsopts=-N
    dependencies:
    - db-lesson
    - time
    - sqlite-simple
    default-extensions:
    - OverloadedStrings

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

コード

app/Main.hs

module Main where

-- import Control.Applicative
import Data.Time
import Database.SQLite.Simple
-- import Database.SQLite.Simple.FromRow
import Lib

data Tool = Tool
  { toolId :: Int,
    name :: String,
    description :: String,
    lastReturned :: Day,
    timesBorrowed :: Int
  }

data User = User
  { userId :: Int,
    userName :: String
  }

instance Show User where
  show user =
    mconcat
      [ show $ userId user,
        ".) ",
        userName user
      ]

instance Show Tool where
  show tool =
    mconcat
      [ show $ toolId tool,
        ".) ",
        name tool,
        "\n description: ",
        description tool,
        "\n last returned: ",
        show $ lastReturned tool,
        "\n times borrowed: ",
        show $ timesBorrowed tool,
        "\n"
      ]

withConn :: String -> (Connection -> IO ()) -> IO ()
withConn dbName action = do
  conn <- open dbName
  action conn
  close conn

addUser :: String -> IO ()
addUser userName =
  withConn
    "tools.db"
    $ \conn -> do
      execute
        conn
        "INSERT INTO users (username) VALUES (?)"
        $ Only userName
      print "user added"

main :: IO ()
main = print "db-lesson"

入出力結果(Terminal, Zsh)

% stack build
db-lesson-0.1.0.0: unregistering (local file changes: app/Main.hs)
db-lesson> build (lib + exe)
Preprocessing library for db-lesson-0.1.0.0..
Building library for db-lesson-0.1.0.0..
Preprocessing executable 'db-lesson-exe' for db-lesson-0.1.0.0..
Building executable 'db-lesson-exe' for db-lesson-0.1.0.0..
[2 of 2] Compiling Main
Linking .stack-work/dist/x86_64-osx/Cabal-3.4.1.0/build/db-lesson-exe/db-lesson-exe ...
db-lesson> copy/register
Installing library in /Users/…/db-lesson/.stack-work/install/x86_64-osx/ab5b3640cd39493075c57a4e40b7505b193ab716e242cc230234f0b245ae2353/9.0.2/lib/x86_64-osx-ghc-9.0.2/db-lesson-0.1.0.0-2ySc30kDbQ7mjauu4huJo
Installing executable db-lesson-exe in /Users/…/db-lesson/.stack-work/install/x86_64-osx/ab5b3640cd39493075c57a4e40b7505b193ab716e242cc230234f0b245ae2353/9.0.2/bin
Registering library for db-lesson-0.1.0.0..
% stack exec db-lesson-exe    
"db-lesson"
%