実践Haskell Aesonを使ったJSONデータの処理 データ型をFromJSONとToJSONのインスタンスにする 独自に記述する parseJSON関数
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)、LESSON 40(Aesonを使ったJSONデータの処理)、40.3(データ型をFromJSONとToJSONのインスタンスにする)、FromJSONとToJSONのインスタンスを独自に記述する、クイックチェック 40-3の解答を求めてみる。
json-lesson/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
- text
- aeson
- bytestring
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:
json-lesson-exe:
main: Main.hs
source-dirs: app
ghc-options:
- -threaded
- -rtsopts
- -with-rtsopts=-N
dependencies:
- json-lesson
tests:
json-lesson-test:
main: Spec.hs
source-dirs: test
ghc-options:
- -threaded
- -rtsopts
- -with-rtsopts=-N
dependencies:
- json-lesson
default-extensions:
- OverloadedStrings
json-lesson/json-lesson.cabal
cabal-version: 1.12
-- This file has been generated from package.yaml by hpack version 0.34.4.
--
-- see: https://github.com/sol/hpack
name: json-lesson
version: 0.1.0.0
description: Please see the README on GitHub at <https://github.com/githubuser/json-lesson#readme>
homepage: https://github.com/githubuser/json-lesson#readme
bug-reports: https://github.com/githubuser/json-lesson/issues
author: Author name here
maintainer: example@example.com
copyright: 2022 Author name here
license: BSD3
license-file: LICENSE
build-type: Simple
extra-source-files:
README.md
CHANGELOG.md
source-repository head
type: git
location: https://github.com/githubuser/json-lesson
library
exposed-modules:
Lib
other-modules:
Paths_json_lesson
hs-source-dirs:
src
default-extensions:
OverloadedStrings
ghc-options: -Wall -Wcompat -Widentities -Wincomplete-record-updates -Wincomplete-uni-patterns -Wmissing-export-lists -Wmissing-home-modules -Wpartial-fields -Wredundant-constraints
build-depends:
aeson
, base >=4.7 && <5
, bytestring
, text
default-language: Haskell2010
executable json-lesson-exe
main-is: Main.hs
other-modules:
Paths_json_lesson
hs-source-dirs:
app
default-extensions:
OverloadedStrings
ghc-options: -Wall -Wcompat -Widentities -Wincomplete-record-updates -Wincomplete-uni-patterns -Wmissing-export-lists -Wmissing-home-modules -Wpartial-fields -Wredundant-constraints -threaded -rtsopts -with-rtsopts=-N
build-depends:
aeson
, base >=4.7 && <5
, bytestring
, json-lesson
, text
default-language: Haskell2010
test-suite json-lesson-test
type: exitcode-stdio-1.0
main-is: Spec.hs
other-modules:
Paths_json_lesson
hs-source-dirs:
test
default-extensions:
OverloadedStrings
ghc-options: -Wall -Wcompat -Widentities -Wincomplete-record-updates -Wincomplete-uni-patterns -Wmissing-export-lists -Wmissing-home-modules -Wpartial-fields -Wredundant-constraints -threaded -rtsopts -with-rtsopts=-N
build-depends:
aeson
, base >=4.7 && <5
, bytestring
, json-lesson
, text
default-language: Haskell2010
コード
json-lesson/app/Main.hs
module Main (main) where
import Data.Aeson
( decode,
eitherDecode,
)
import Data.ByteString.Lazy as B (ByteString)
import Lib (Name (..))
json :: B.ByteString
json = "{\"firstName\": \"first name\", \"lastName\": \"last name\"}"
decoded :: Maybe Name
decoded = decode json
eitherDecoded :: Either String Name
eitherDecoded = eitherDecode json
wrongJson :: B.ByteString
wrongJson = "{}"
wrongDecoded :: Maybe Name
wrongDecoded = decode wrongJson
wrongEitherDecoded :: Either String Name
wrongEitherDecoded = eitherDecode wrongJson
main :: IO ()
main = do
print json
print decoded
print eitherDecoded
print wrongJson
print wrongDecoded
print wrongEitherDecoded
json-lesson/src/Lib.hs
module Lib
( Name (..),
)
where
import Data.Aeson (FromJSON (parseJSON), Value (Object), (.:))
import Data.Text as T (Text)
data Name = Name
{ firstName :: T.Text,
lastName :: T.Text
}
deriving (Show)
instance FromJSON Name where
parseJSON (Object v) =
Name
<$> v
.: "firstName"
<*> v
.: "lastName"
parseJSON _ = error ""
入出力結果(Terminal, Zsh)
% stack exec json-lesson-exe
"{\"firstName\": \"first name\", \"lastName\": \"last name\"}"
Just (Name {firstName = "first name", lastName = "last name"})
Right (Name {firstName = "first name", lastName = "last name"})
"{}"
Nothing
Left "Error in $: key \"firstName\" not found"
%