重叠的aeson实例

Question

在安装新版本的伊森之后(ghc-pkg list | grep aeson显示伊森-1.4.7.1)，我现在有一个以下错误：

RoseTree.hs:17:69: error:
    • Overlapping instances for aeson-1.2.1.0:Data.Aeson.Types.ToJSON.RecordToPairs
                                  JSON.Value
                                  (dlist-1.0:Data.DList.Internal.DList
                                     aeson-1.2.1.0:Data.Aeson.Types.Internal.Pair)
                                  JSON.Zero
                                  (M1
                                     S
                                     ('MetaSel
                                        ('Just "value")
                                        'NoSourceUnpackedness
                                        'NoSourceStrictness
                                        'DecidedLazy)
                                     (Rec0 a))
        arising from the 'deriving' clause of a data type declaration
      Matching instances:
        two instances involving out-of-scope types
        (use -fprint-potential-instances to see them all)
      (The choice depends on the instantiation of ‘a’
       To pick the first instance above, use IncoherentInstances
       when compiling the other instance declarations)
    • When deriving the instance for (JSON.ToJSON (RoseTree a))
   |
17 |                    deriving (Show, Generic, Functor, JSON.FromJSON, JSON.ToJSON)
   |                                                                     ^^^^^^^^^^^
Failed, one module loaded.

我的源码如下：

{-# LANGUAGE DeriveAnyClass      #-}
{-# LANGUAGE DeriveFunctor       #-}
{-# LANGUAGE DeriveGeneric       #-}
{-# LANGUAGE FlexibleInstances   #-}
{-# LANGUAGE OverloadedStrings   #-}
{-# LANGUAGE IncoherentInstances #-}

module RoseTree where

import qualified Data.Aeson   as JSON
import           Data.Maybe
import           Data.Monoid
import           Data.Tree
import           GHC.Generics (Generic)

data RoseTree a = RoseTree { value :: a, children :: [RoseTree a]}
                   deriving (Show, Generic, Functor, JSON.FromJSON, JSON.ToJSON)

如何摆脱这个错误？提前谢谢。

Answer 1

事实证明，cabal被早期版本的aeson 1.2.1.0卡住了，通过普通方法修复它没有任何帮助。问题可以通过切换到Nix来解决。