Hasseell ADT与aeson [英] Haskell ADTs with aeson

问题描述

我一直在与一个简单的ADT斗争，试图让它来回往返于JSON，但是我没有运气，不管我如何尝试按摩或修改类型。我错过了什么？

编译时，我总是得到相同的运行时错误：

 >让t =华氏温度
> fromJSON $ toJSON t 
错误期待遇到（），遇到Object而不是
  

尝试这只是给了我没有，大概是因为相同的错误：解码$编码t

我试图遵循这些来源，但我似乎无法解决这个运行时错误，无论我尝试什么：
Haskell :: Aeson ::根据字段值解析ADT
https://www.fpcomplete.com/user/Geraldus/algebraic-data-types-adts-with-aeson

<这是我使用的一种代码形式。起初，我试图将它用作嵌入另一种类型的类型，但是当它不起作用时，我添加了value键来尝试对此进行解析（没有运气）。

  data TemperatureType =摄氏度
 |华氏度
导出（Show，Read，Typeable，Data，Eq）
 
  - 这不起作用
  -  $（deriveJSON defaultOptions''TemperatureType）
 
实例ToJSON TemperatureType其中
 toJSON Fahrenheit = object [value。= StringFahrenheit] 
 toJSON Celsius = object [value。= StringCelsius] 
 
实例FromJSON TemperatureType其中
 parseJSON（Object x）= toTemperatureType< $> x。：value
 
 toTemperatureType :: Text  - >温度类型
 toTemperatureType华氏度=华氏温度
温度类型摄氏度=摄氏温度
  

解决方案

Haskell需要关于您的表达式结果类型的帮助，因为在当前调用中无法推断它：

 > fromJSON $ toJSON t :: Result TemperatureType 
  

I've been fighting with a simple ADT, trying to get it to round-trip back and forth to JSON, but I've had no luck, no matter how I try to massage or modify the type. What am I missing?

When it compiles, I always get the same runtime error:

> let t = Fahrenheit
> fromJSON $ toJSON t
Error "when expecting a (), encountered Object instead"

Trying this just gives me "Nothing", presumably because of the same error: decode $ encode t

I've tried to follow these sources, but I can't seem to get around this runtime error, no matter what I try: Haskell :: Aeson :: parse ADT based on field value https://www.fpcomplete.com/user/Geraldus/algebraic-data-types-adts-with-aeson

Here's one form of the code I'm using. At first, I tried to use this as a type embedded in another type, but when that didn't work I added the "value" key to try to make parsing of this easier (no luck).

data TemperatureType = Celsius
                     | Fahrenheit
                     deriving (Show,Read,Typeable,Data,Eq)

-- This doesn't work either
-- $(deriveJSON defaultOptions ''TemperatureType)

instance ToJSON TemperatureType where
   toJSON Fahrenheit = object [ "value" .= String "Fahrenheit" ]
   toJSON Celsius    = object [ "value" .= String "Celsius" ]

instance FromJSON TemperatureType where
    parseJSON (Object x) = toTemperatureType <$> x .: "value"

toTemperatureType :: Text -> TemperatureType
toTemperatureType "Fahrenheit" = Fahrenheit
toTemperatureType "Celsius"    = Celsius

解决方案

Haskell need help from you about the type of your expression result since in the current call it's not possible to infer it:

> fromJSON $ toJSON t :: Result TemperatureType

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