解码具有多个带有分隔标签的构造器的对象 [英] Decoding object with multiple constuctors with separated tags

问题描述

我有成对的两个字符串形式的数据，其中第一个是识别作为第二个JSON传递的JSON形状的密钥.

I have data in form of pairs of two strings, where first is the key identifying shape of the JSON delivered as the second one.

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，基于fopooo，baasdasda1等:

I would like to parse it to same data type, based on the fopooo, baasdasda1, etc :

data Test
  = Foo
    { a :: Int
    , b :: Int
    }
  | Bar
    { a :: Int
    }
  deriving (Show, Generic)

在Aeson中有一个标签功能，但是它似乎需要在对象内部存在标签.

In the Aeson there is a tag feature, but it seems to require presence of the tag inside the object.

是否有某种处理方式(带有外部标签)

magicDecode :: String -> Test
magicDecode "fooooo" = decodeSpecyfic Foo
magicDecode "barrrr" = decodeSpecyfic Bar

还是类似的东西?

我为此屈从于处理带有记录的类型

I've currenlty mananged to handle types with records by this

test :: [(String, Text)]
test =
  [ ("foooo", "Foo")
  , ("barrr", "Bar")
  , ("aaaa", "Lel")
  ]

dec :: String -> ByteString -> Test
dec t x =
  fromJust $
  parseMaybe
    (genericParseJSON defaultOptions {sumEncoding = ObjectWithSingleField})
    (object [fromJust (lookup t test) .= fromJust (decode x :: Maybe Value)])

(问题是如果您不是构造函数参数:()

(Issue is if thee are no constructor params :( )

推荐答案

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

文档解释了有关UntaggedValue的信息:

As the documentation explains about UntaggedValue:

解码时，按定义顺序尝试构造函数.如果某些编码重叠，则定义的第一个编码将成功.

When decoding, constructors are tried in the order of definition. If some encodings overlap, the first one defined will succeed.

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，您不应该使用记录来建模求和类型，因为记录访问器是部分访问的:

That said, you shouldn't model sum types with records, because the record accessors are partial:

*Q59916344 Q59916344> b (Foo 42 1337)
1337
*Q59916344 Q59916344> b (Bar 42)
*** Exception: No match in record selector b

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