Haskell语言学习笔记（66）Aeson

Data.Aeson

安装 aeson

$ cabal install aeson

Installed aeson-1.2.3.0

Prelude> :m +Data.Aeson

Prelude Data.Aeson>

Data.Aeson 是一个处理 JSON 数据的库。

处理 AST

Prelude Data.Aeson> decode "{\"foo\": 123}" :: Maybe Value

Just (Object (fromList [("foo",Number 123.0)]))

Prelude Data.Aeson> decode "{\"foo\": [\"abc\",\"def\"]}" :: Maybe Value

Just (Object (fromList [("foo",Array [String "abc",String "def"])]))

Prelude Data.Aeson> decode "[1,2,3]" :: Maybe [Int]

Just [1,2,3]

Prelude Data.Aeson> decode "{\"foo\":1,\"bar\":2}" :: Maybe (Data.Map.Map String Int)

Just (fromList [("bar",2),("foo",1)])

处理 Person（1）

{-# LANGUAGE DeriveGeneric #-}

import GHC.Generics

import Data.Aeson

import Data.Text

data Person = Person {

      name :: Text

    , age  :: Int

    } deriving (Generic, Show)

data Persons = Persons {

      persons :: [Person]

    } deriving (Generic, Show)

instance ToJSON Person

instance FromJSON Person

instance ToJSON Persons

instance FromJSON Persons

*Main> :set -XOverloadedStrings

*Main> encode (Person {name = "Joe", age = 12})

"{\"name\":\"Joe\",\"age\":12}"

*Main> decode "{\"name\":\"Joe\",\"age\":12}" :: Maybe Person

Just (Person {name = "Joe", age = 12})

*Main> encode (Persons {persons = [Person {name = "Joe", age = 12}]})

"{\"persons\":[{\"name\":\"Joe\",\"age\":12}]}"

*Main> decode "{\"persons\":[{\"name\":\"Joe\",\"age\":12}]}" :: Maybe Persons

Just (Persons {persons = [Person {name = "Joe", age = 12}]})

• instance ToJSON Persons
  
  要将 Persons 类型的对象序列化成 JSON 字符串，Persons 类型必须是 ToJSON 类型类的实例
• instance FromJSON Persons
  
  要将 JSON 字符串反序列化成 Persons 类型的对象，Persons 类型必须是 FromJSON 类型类的实例

处理 Person（2）

{-# LANGUAGE OverloadedStrings #-}

import Data.Aeson

import Data.Text

data Person = Person {

      name :: Text

    , age  :: Int

    } deriving Show

data Persons = Persons {

      persons :: [Person]

    } deriving Show

instance FromJSON Person where

    parseJSON = withObject "Person" $ \v -> Person

        <$> v .: "name"

        <*> v .: "age"

instance ToJSON Person where

    toJSON (Person name age) =

        object ["name" .= name, "age" .= age]

instance FromJSON Persons where

    parseJSON = withObject "Persons" $ \v -> Persons

        <$> v .: "persons"

instance ToJSON Persons where

    toJSON (Persons persons) =

        object ["persons" .= persons]

*Main> :set -XOverloadedStrings

*Main> encode (Person {name = "Joe", age = 12})

"{\"name\":\"Joe\",\"age\":12}"

*Main> decode "{\"name\":\"Joe\",\"age\":12}" :: Maybe Person

Just (Person {name = "Joe", age = 12})

*Main> encode (Persons {persons = [Person {name = "Joe", age = 12}]})

"{\"persons\":[{\"name\":\"Joe\",\"age\":12}]}"

*Main> decode "{\"persons\":[{\"name\":\"Joe\",\"age\":12}]}" :: Maybe Persons

Just (Persons {persons = [Person {name = "Joe", age = 12}]})

处理 Person（3）

{-# LANGUAGE OverloadedStrings #-}

import Data.Aeson

import Data.Text

data Person = Person {

      name :: Text

    , age  :: Int

    } deriving Show

instance FromJSON Person where

    parseJSON = withObject "Person" $ \v -> Person

        <$> v .: "name"

        <*> v .:? "age" .!= 20

*Main> :set -XOverloadedStrings

*Main> decode "{\"name\":\"Joe\"}" :: Maybe Person

Just (Person {name = "Joe", age = 20})

处理 Person（4）

{-# LANGUAGE DeriveGeneric #-}

import GHC.Generics

import Data.Aeson

import Data.Text (Text)

data Person = Person {

      field_NAME :: Text

    , field_AGE  :: Int

    } deriving (Generic, Show)

customOptions = defaultOptions

                { fieldLabelModifier = drop $ length ("field_" :: String)

                }

instance ToJSON Person where

    toJSON = genericToJSON customOptions

instance FromJSON Person where

    parseJSON = genericParseJSON customOptions

*Main> :set -XOverloadedStrings

*Main> encode (Person {field_NAME = "Joe", field_AGE = 12})

"{\"AGE\":12,\"NAME\":\"Joe\"}"

*Main> decode "{\"NAME\":\"Joe\",\"AGE\":12}" :: Maybe Person

Just (Person {field_NAME = "Joe", field_AGE = 12})