Haskell 非法派生项泛型

Question

{-# LANGUAGE DataKinds         #-}
{-# LANGUAGE DeriveGeneric     #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE TypeOperators     #-}

import qualified Data.ByteString.Lazy    as BL
import           Data.Csv                (DefaultOrdered, FromRecord,
                                          ToNamedRecord, ToRecord)
import           Data.Generics
import           Data.Proxy
import qualified Data.Text               as T
import           Network.HTTP.Client     (newManager)
import           Network.HTTP.Client.TLS (tlsManagerSettings)
import           Servant.API
import           Servant.Client
import           Servant.CSV.Cassava
import           System.Environment

data Cata = Cata
  {
    csvDate       :: String,
    csvOpen       :: String,
    csvHigh       :: String,
    csvLow        :: String,
    csvClose      :: String,
    csvVolume     :: String,
    csvExDividend :: String,
    csvSplitRatio :: String,
    csvAdjOpen    :: String,
    csvAdjHigh    :: String,
    csvAdjLow     :: String,
    csvAdjClose   :: String,
    csvAdjVolume  :: String
  } deriving (Show, Generic)

instance DefaultOrdered Cata
instance FromRecord Cata
instance ToNamedRecord Cata
instance ToRecord Cata

我编译上面的代码并得到以下错误:为什么？

$ stack ghc servantcsv.hs
[1 of 1] Compiling Main             ( servantcsv.hs, servantcsv.o )

servantcsv.hs:34:21: error:
    • Illegal deriving item ‘Generic’
    • In the data declaration for ‘Cata’
   |
34 |   } deriving (Show, Generic)
   |                     ^^^^^^^

GHC 版本:ghc-8.4.3

Answer 1

您想要导入GHC.Generics而不是Data.Generics。您导入的内容确实具有 Generic 类型，但它不是一个类:

type Generic c = forall a. Data a => a -> c a

当然，这不能成为“派生”子句的一部分。