実践Haskell Aesonを使ったJSONデータの処理 FromJSONクラス、ToJSONクラス、Generic

入門Haskellプログラミング
 楽天ブックス
Yahoo!ショッピング
au PAY マーケット

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UNIT7(実践Haskell)、LESSON40(Aesonを使ったJSONデータの処理)、40.6(練習問題)Q40-2の解答を求めてみる。

package.yaml

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

dependencies:
- base >= 4.7 && < 5
library:
  source-dirs: src

executables:
  json-lesson-exe:
    main:                Main.hs
    source-dirs:         app
    ghc-options:
    - -threaded
    - -rtsopts
    - -with-rtsopts=-N
    dependencies:
    - json-lesson
    - aeson
    - bytestring
    # - text
    default-extensions:
    - OverloadedStrings
    - DeriveGeneric

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

コード

app/Main.hs

module Main where

import Data.Aeson
-- import qualified Data.ByteString as T
-- import Data.ByteString.Lazy as B
import Data.ByteString.Lazy.Char8 as BC
-- import Data.Text as T
-- import qualified Data.Text.Encoding as E
import GHC.Generics (Generic)
import Lib ()

data IntList
  = EmptyList
  | Cons Int IntList
  deriving (Show, Generic)

instance FromJSON IntList

instance ToJSON IntList

intListExample :: IntList
intListExample =
  Cons 1 $
    Cons 2 $ EmptyList

josnExample :: BC.ByteString
josnExample = encode intListExample

decodedExample :: Maybe IntList
decodedExample = decode josnExample

errDecoded :: Maybe IntList
errDecoded = decode "{}"

main :: IO ()
main = do
  print intListExample
  print josnExample
  print decodedExample
  print errDecoded

入出力結果(Terminal, Zsh)

% stack build
json-lesson> configure (lib + exe)
Configuring json-lesson-0.1.0.0...
json-lesson> build (lib + exe)
Preprocessing library for json-lesson-0.1.0.0..
Building library for json-lesson-0.1.0.0..
Preprocessing executable 'json-lesson-exe' for json-lesson-0.1.0.0..
Building executable 'json-lesson-exe' for json-lesson-0.1.0.0..
[1 of 2] Compiling Paths_json_lesson [flags changed]
[2 of 2] Compiling Main
Linking .stack-work/dist/x86_64-osx/Cabal-3.4.1.0/build/json-lesson-exe/json-lesson-exe ...
json-lesson> copy/register
Installing library in /Users/…/json-lesson/.stack-work/install/x86_64-osx/517038c0363b4c0a7ccbe3fd94330b03ef892e9dc18133de80a96d8183f01a77/9.0.2/lib/x86_64-osx-ghc-9.0.2/json-lesson-0.1.0.0-DuCC7x37YWa5QrL5HPMKvq
Installing executable json-lesson-exe in /Users/…/json-lesson/.stack-work/install/x86_64-osx/517038c0363b4c0a7ccbe3fd94330b03ef892e9dc18133de80a96d8183f01a77/9.0.2/bin
Registering library for json-lesson-0.1.0.0..
% stack exec json-lesson-exe
Cons 1 (Cons 2 EmptyList)
"{\"contents\":[1,{\"contents\":[2,{\"tag\":\"EmptyList\"}],\"tag\":\"Cons\"}],\"tag\":\"Cons\"}"
Just (Cons 1 (Cons 2 EmptyList))
Nothing
%