計算機科学のブログ

name:                json-lesson
version:             0.1.0.0
github:              "githubuser/json-lesson"
license:             BSD-3-Clause
author:              "Author name here"
maintainer:          "example@example.com"
copyright:           "2025 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
- aeson
- bytestring
- text

default-extensions:
- OverloadedStrings

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

module Main (main) where

-- import Lib
import qualified Data.Text as T
import qualified Data.ByteString.Lazy as B
import Data.Aeson

data Name = Name {
    firstName :: T.Text,
    lastName :: T.Text
    } deriving (Show)

instance FromJSON Name where
    parseJSON (Object v) = Name <$> v.: "firstName" <*> v.: "lastName"

instance ToJSON Name where
    toJSON (Name fname lname) = object [
        "firstName" .= fname,
        "lastName" .= lname
        ]

name :: Name
name = Name {firstName = "first name", lastName = "last name"}

encoded :: B.ByteString
encoded = encode name

decoded :: Maybe Name
decoded = decode encoded

eitherDecoded :: Either String Name
eitherDecoded = eitherDecode encoded

errorRawJson :: B.ByteString
errorRawJson = "{\"firstName\":\"first name\",\"lastName\"}"

errorDecoded :: Maybe Name
errorDecoded = decode errorRawJson

errorEitherDecoded :: Either String Name
errorEitherDecoded = eitherDecode errorRawJson

main :: IO ()
main = do
    print name
    print encoded
    print decoded
    print eitherDecoded
    print errorRawJson
    print errorDecoded
    print errorEitherDecoded

% stack run         
json-lesson-0.1.0.0: unregistering (local file changes: app/Main.hs)
json-lesson> build (lib + exe) with ghc-9.8.4
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 Main [Source file changed]

/Users/.../json-lesson/app/Main.hs:14:5: warning: [GHC-62161] [-Wincomplete-patterns]
    Pattern match(es) are non-exhaustive
    In an equation for ‘parseJSON’:
        Patterns of type ‘Value’ not matched:
            Array _
            String _
            Number _
            Bool _
            ...
   |
14 |     parseJSON (Object v) = Name <$> v.: "firstName" <*> v.: "lastName"
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[3 of 3] Linking .stack-work/dist/aarch64-osx/ghc-9.8.4/build/json-lesson-exe/json-lesson-exe [Objects changed]
ld: warning: -U option is redundant when using -undefined dynamic_lookup
json-lesson> copy/register
Installing library in /Users/.../json-lesson/.stack-work/install/aarch64-osx/27e00dd04a6f7fc061662d4eebb16cc69859af4840b4ab18e626ce23aacafb60/9.8.4/lib/aarch64-osx-ghc-9.8.4/json-lesson-0.1.0.0-5hrW8Xn7EXr6hNkAQFVUwP
Installing executable json-lesson-exe in /Users/.../json-lesson/.stack-work/install/aarch64-osx/27e00dd04a6f7fc061662d4eebb16cc69859af4840b4ab18e626ce23aacafb60/9.8.4/bin
Registering library for json-lesson-0.1.0.0..
Name {firstName = "first name", lastName = "last name"}
"{\"firstName\":\"first name\",\"lastName\":\"last name\"}"
Just (Name {firstName = "first name", lastName = "last name"})
Right (Name {firstName = "first name", lastName = "last name"})
"{\"firstName\":\"first name\",\"lastName\"}"
Nothing
Left "Unexpected \"}\", expecting :"
%