Haskell非法衍生品泛型

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

I compile the above code and get the following error: why?

$ 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 Version: ghc-8.4.3

Answer 1

You want to import GHC.Generics not Data.Generics . What you have imported does have a type Generic but it isn't a class:

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

Certainly that can't be part of a "deriving" clause.