Haskell Aeson return 空对象
Haskell Aeson return empty object
我正在尝试 return 如果不是 Nothing,则 JSON 数据表示,如果 Nothing,则为空的 JSON 对象;
我知道我能做到:
encode ()
-- "[]"
但现在我想要一个空对象 ("{}"
)。
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}"
但最终没有人会用 Nothing 表示,如果我这样做 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)
encode Nothing
将始终 return null
。可以通过 encode (object [])
对空对象进行编码。如果您想以这种方式编码 Nothings
,您可以为 Maybe
值编写自定义编码函数,如下所示。
encodeMaybe :: ToJSON a => Maybe a -> ByteString
encodeMaybe (Just x) = encode x
encodeMaybe Nothing = encode (object [])
或者
toJSONMaybe :: ToJSON a => Maybe a -> Value
toJSONMaybe (Just x) = toJSON x
toJSONMaybe Nothing = object []
我正在尝试 return 如果不是 Nothing,则 JSON 数据表示,如果 Nothing,则为空的 JSON 对象;
我知道我能做到:
encode ()
-- "[]"
但现在我想要一个空对象 ("{}"
)。
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}"
但最终没有人会用 Nothing 表示,如果我这样做 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)
encode Nothing
将始终 return null
。可以通过 encode (object [])
对空对象进行编码。如果您想以这种方式编码 Nothings
,您可以为 Maybe
值编写自定义编码函数,如下所示。
encodeMaybe :: ToJSON a => Maybe a -> ByteString
encodeMaybe (Just x) = encode x
encodeMaybe Nothing = encode (object [])
或者
toJSONMaybe :: ToJSON a => Maybe a -> Value
toJSONMaybe (Just x) = toJSON x
toJSONMaybe Nothing = object []