Haskell Aeson返回空对象

Question

我试图返回一个JSON数据表示，如果不是Nothing，则返回一个空JSON对象；

我知道我可以做到：

encode ()
-- "[]"

但是现在我想要有一个空对象("{}")。

我有这个，它可以根据给定的字段生成JSON：

λ data Person = Person { id :: Integer, height :: Float } deriving (Show)
λ instance ToJSON Person where toJSON (Person { id = id, height = height }) = object [ "id" .= id, "height" .= height ]
λ encode (Person 1 72.8)
-- "{\"height\":72.8,\"id\":1}"

但最终一个人的缺席将被表示为没有任何东西，如果我执行encode (Nothing)，我会得到一个错误：

<interactive>:11:1: error:
    • Ambiguous type variable ‘a0’ arising from a use of ‘encode’
      prevents the constraint ‘(ToJSON a0)’ from being solved.
      Probable fix: use a type annotation to specify what ‘a0’ should be.
      These potential instances exist:
        instance ToJSON DotNetTime
          -- Defined in ‘aeson-1.4.7.1:Data.Aeson.Types.ToJSON’
        instance ToJSON Value
          -- Defined in ‘aeson-1.4.7.1:Data.Aeson.Types.ToJSON’
        instance (ToJSON a, ToJSON b) => ToJSON (Either a b)
          -- Defined in ‘aeson-1.4.7.1:Data.Aeson.Types.ToJSON’
        ...plus 26 others
        ...plus 63 instances involving out-of-scope types
        (use -fprint-potential-instances to see them all)
    • In the expression: encode (Nothing)
      In an equation for ‘it’: it = encode (Nothing)

Answer 1

encode Nothing将始终返回null。编码一个空对象可以通过执行encode (object [])来完成。如果您想以这种方式编码Nothings，您可以为Maybe值编写一个自定义编码函数，如下所示。

encodeMaybe :: ToJSON a => Maybe a -> ByteString
encodeMaybe (Just x) = encode x
encodeMaybe Nothing  = encode (object [])

或者另选地

toJSONMaybe :: ToJSON a => Maybe a -> Value
toJSONMaybe (Just x) = toJSON x
toJSONMaybe Nothing  = object []