如何编写“派生实例”声明？

Question

我正在尝试将加密哈希存储为记录类型中的字段，但编译器抱怨：

HashTest.hs:13:1: error:
• No instance for (Data (Digest MD5))
    arising from the second field of ‘MyRecord’ (type ‘Digest MD5’)
  Possible fix:
    use a standalone 'deriving instance' declaration,
      so you can specify the instance context yourself
• When deriving the instance for (Data MyRecord)
   |
13 |                          } deriving (Show, Eq, Data, Generic)
   |                                                ^^^^

代码如下：

{-# LANGUAGE DeriveGeneric         #-}
{-# LANGUAGE DeriveDataTypeable    #-}

module Playpen.HashTest.HashTest where

import Data.Generics (Data, Typeable)
import GHC.Generics (Generic)

import Crypto.Hash

data MyRecord = MyRecord { idx :: Int
                         , sig :: Digest MD5
                         } deriving (Show, Eq, Generic)

如果我编写一个独立的“派生实例”子句如下...

{-# LANGUAGE DeriveGeneric         #-}
{-# LANGUAGE DeriveDataTypeable    #-}
{-# LANGUAGE StandaloneDeriving    #-}

module Playpen.HashTest.HashTest where

import Data.Generics (Data, Typeable)
import GHC.Generics (Generic)

import Crypto.Hash

data MyRecord = MyRecord { idx :: Int
                         , sig :: Digest MD5
                         } deriving (Show, Eq, Generic)

deriving instance Data MyRecord

...那么错误就变成了

HashTest.hs:16:1: error:
    • No instance for (Data (Digest MD5)) arising from a use of ‘k’
    • In the expression: ((z MyRecord `k` a1) `k` a2)
      In an equation for ‘Data.Data.gfoldl’:
          Data.Data.gfoldl k z (MyRecord a1 a2)
            = ((z MyRecord `k` a1) `k` a2)
      When typechecking the code for ‘Data.Data.gfoldl’
        in a derived instance for ‘Data MyRecord’:
        To see the code I am typechecking, use -ddump-deriv
      In the instance declaration for ‘Data MyRecord’
   |
16 | deriving instance Data MyRecord
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

HashTest.hs:16:1: error:
    • No instance for (Data (Digest MD5)) arising from a use of ‘k’
    • In the expression: k (k (z MyRecord))
      In an equation for ‘Data.Data.gunfold’:
          Data.Data.gunfold k z _ = k (k (z MyRecord))
      When typechecking the code for ‘Data.Data.gunfold’
        in a derived instance for ‘Data MyRecord’:
        To see the code I am typechecking, use -ddump-deriv
      In the instance declaration for ‘Data MyRecord’
   |
16 | deriving instance Data MyRecord
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

谁能帮我理解这个错误以及如何解决它？

Answer 1

一种选择是跳过Data 和Generic 实例，直到确实需要它们为止；然后在那时充分了解Data 和Generic 以手动实现它们。

Answer 2

要为您的类型派生Data 实例，它包含的所有类型也应该具有该实例。

使Digest MD5 也成为Data 的实例。