Haskell/GHC 泛型文档示例缺少类型类约束？

Question

我正在按照教程为来自hackage documentation 的GHC.Generics 的数据类型构建通用“编码”函数。我已将代码复制粘贴到（并包括）“包装器和通用默认值”部分，但出现以下错误：

Main.hs:35:14:
    Could not deduce (Encode' (Rep a)) arising from a use of ‘encode'’
    from the context (Encode a)
      bound by the class declaration for ‘Encode’
      at Main.hs:(32,1)-(35,29)
    or from (Generic a)
      bound by the type signature for encode :: Generic a => a -> [Bool]
      at Main.hs:33:13-23
    In the expression: encode' (from x)
    In an equation for ‘encode’: encode x = encode' (from x)
Failed, modules loaded: none.

我复制的代码如下：

{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE DefaultSignatures #-}
{-# LANGUAGE FlexibleContexts  #-}
{-# LANGUAGE DeriveAnyClass    #-}
module Main where

import GHC.Generics

class Encode' f where
  encode' :: f p -> [Bool]

instance Encode' V1 where
  encode' x = undefined

instance Encode' U1 where
  encode' U1 = []

instance (Encode' f, Encode' g) => Encode' (f :+: g) where
  encode' (L1 x) = False : encode' x
  encode' (R1 x) = True  : encode' x

instance (Encode' f, Encode' g) => Encode' (f :*: g) where
  encode' (x :*: y) = encode' x ++ encode' y

instance (Encode c) => Encode' (K1 i c) where
  encode' (K1 x) = encode x

instance (Encode' f) => Encode' (M1 i t f) where
  encode' (M1 x) = encode' x

class Encode a where
  encode :: a -> [Bool]
  default encode :: (Generic a) => a -> [Bool]
  encode x = encode' (from x)

我认为问题在于最终的类声明 (class Encode a where ...)。我已经通过添加一个额外的约束来修复它，以获得这个：

class Encode a where
  encode :: a -> [Bool]                                                                                                                                                              
  default encode :: (Generic a, Encode' (Rep a)) => a -> [Bool]
  encode x = encode' (from x)

这似乎像宣传的那样工作——我可以声明新的数据类型并使用DeriveAnyClass 和deriving 使它们可编码。但是，我不确定为什么我的修复是必要的。我的问题是：

• 文档有误吗？
• Encode' (Rep a) 约束是否应该出现在有关 hackage 的示例代码中？
• 如果没有，我应该添加什么才能使代码正常工作？

Answer 1

是的，我认为文档是错误的。约束是必需的。