我有表格的数据
fooooo
{"a": 123}
barrrr
{"a": 123, "b": 123}
fooooo
{"a": 123}
我想将其解析为相同的数据类型:
data Test
= Foo
{ a :: Int
, b :: Int
}
| Bar
{ a :: Int
}
deriving (Show, Generic)
在Aeson中有一个tag feature,但似乎需要在对象内部存在标签。
是否有某种方式可以处理它(带标签在外面)
magicDecode :: String -> Test
magicDecode "fooooo" = decodeSpecyfic Foo
magicDecode "barrrr" = decodeSpecyfic Bar
或类似的东西?
您可以使用UntaggedValue
选项:
{-# LANGUAGE DeriveGeneric #-}
module Q59916344 where
import Data.Aeson
import GHC.Generics
myOptions = defaultOptions { sumEncoding = UntaggedValue }
data Test = Foo { a :: Int, b :: Int } | Bar { a :: Int } deriving (Show, Generic)
instance FromJSON Test where
parseJSON = genericParseJSON myOptions
instance ToJSON Test where
toJSON = genericToJSON myOptions
toEncoding = genericToEncoding myOptions
正如the documentation解释UntaggedValue
:
解码时,按定义顺序尝试构造器。如果某些编码重叠,则定义的第一个编码将成功。
演示:
*Q59916344 Q59916344> decode "{\"a\": 123}" :: Maybe Test
Just (Bar {a = 123})
*Q59916344 Q59916344> decode "{\"a\": 123, \"b\": 123}" :: Maybe Test
Just (Foo {a = 123, b = 123})
也就是说,您不应该用记录来建模求和类型],因为记录访问器是局部的:
*Q59916344 Q59916344> b (Foo 42 1337)
1337
*Q59916344 Q59916344> b (Bar 42)
*** Exception: No match in record selector b