[英]Instance Data.Sequence with Haskell Aeson
I have the next problem with Data.Sequence
and Aeson. 我有Data.Sequence
和Aeson的下一个问题。 I want to create my data DraftVar
deriving Generic
, which use Data.Sequence
on the constructor DV2
. 我想创建我的数据DraftVar
派生Generic
,它在构造函数DV2
上使用Data.Sequence
。
{-# LANGUAGE DeriveGeneric, OverloadedStrings #-}
-- Imports
import Data.Aeson
import GHC.Generics
import qualified Data.Sequence as DS
-- Data
data DraftVar =
DV1 { dv1_val :: Int }
| DV2 { dv2_list :: DS.Seq DraftVar }
deriving (Show, Generic)
instance ToJSON DraftVar
instance FromJSON DraftVar
With this code, I receive and error that says: No instance for (ToJSON (DS.Seq DraftVar))
. 使用此代码,我收到并显示错误: No instance for (ToJSON (DS.Seq DraftVar))
。 So I need to create a instance of Data.Sequence
for the Aeson library. 所以我需要为Aeson库创建一个Data.Sequence
实例。
On the ToJSON
instance declaration, I decide to transform the Data.Sequence
to a list. 在ToJSON
实例声明中,我决定将Data.Sequence
转换为列表。 The code is: 代码是:
import Data.Foldable as DF
instance (Show a) => ToJSON (DS.Seq a) where
toJSON l = object [ "values" .= show (DF.toList l) ]
But what happen, when I want to extract from this Json the list and then transform the data to Data.Sequence
? 但是,当我想从这个Json中提取列表然后将数据转换为Data.Sequence
什么?
instance (Show a) => FromJSON (DS.Seq a) where
parseJSON (Object o) = ???
Maybe I need a library or an special function from Aeson, I don't know. 也许我需要Aeson的图书馆或特殊功能,我不知道。 The most useful example (that I found) is this: Parsing an Array with Haskell Aeson 最有用的例子(我发现)是这样的: 用Haskell Aeson解析一个数组
Do you have a better approach? 你有更好的方法吗?
[]
and Seq
are isomorphic via fromList
/ toList
(they just have vastly different representations and performance characteristics), so the most straightforward way to implement FromJSON
and ToJSON
for Seq
is by re-using the existing implementation for []
: []
和Seq
通过fromList
/ toList
是同构的(它们只有很多不同的表示和性能特征),所以为Seq
实现FromJSON
和ToJSON
最直接的方法是重用[]
的现有实现:
import qualified Data.Sequence as DS
import Data.Foldable as DF
instance (ToJSON a) => ToJSON (DS.Seq a) where
toJSON = toJSON . DF.toList
instance (FromJSON a) => FromJSON (DS.Seq a) where
parseJSON = fmap DS.fromList . parseJSON
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