无法从实例声明的超类中推断出（半群（可选 a））

Question

“Haskell Programming: From first principle”中的以下代码编译失败：

module Learn where
import Data.Semigroup
import Data.Monoid

-- Exercise: Optional Monoid
data Optional a = Nada
                | Only a
                deriving (Eq, Show)

instance Monoid a => Monoid (Optional a) where
 mempty = Nada
 mappend Nada Nada = Nada
 mappend (Only a) Nada = Only $ mappend a mempty
 mappend Nada (Only a) = Only $ mappend mempty a
 mappend (Only a) (Only b) = Only $ mappend a b

它给出了以下错误：

intermission.hs:11:10: error:
    • Could not deduce (Semigroup (Optional a))
        arising from the superclasses of an instance declaration
      from the context: Monoid a
        bound by the instance declaration at intermission.hs:11:10-40
    • In the instance declaration for ‘Monoid (Optional a)’
   |
11 | instance Monoid a => Monoid (Optional a) where
   |   

为了阻止 ghc 抱怨，我必须创建 Optional a 的半群实例并定义“”。这对我来说不太有意义，我想知道我是否忽略了一些东西。

Answer 1

解决办法如下：

import Data.Monoid

data Optional a = Nada | Only a deriving (Eq, Show)

instance Monoid a => Monoid (Optional a) where
   mempty = Nada

instance Semigroup a => Semigroup (Optional a) where
  Nada <> (Only a) = Only a
  (Only a) <> Nada = Only a
  (Only a) <> (Only a') = Only (a <> a')
  Nada <> Nada = Nada

main :: IO ()
main = do
  print $ Only (Sum 1) `mappend` Only (Sum 1)

Answer 2

“NOTE: Semigroup is a superclass of Monoid since base-4.11.0.0.”

其中的超类列表一直在缓慢发展。随着新的有用类的提出，旧类的 API 会更新以反映它们的关系。这具有破坏旧代码的不幸影响。 Base 4.11.1.0 于 2018 年 4 月发布，对 Monoid 进行了重大更改。