haskell - 指半群或Monoid

我尝试解决haskellbook中的练习，并创建了以下模块：

module Exercises where

import Data.Semigroup
import Data.Monoid
import Test.QuickCheck

data Trivial = Trivial deriving (Eq, Show)

instance Semigroup Trivial where
  _ <> _ = Trivial

instance Monoid Trivial where
  mempty = Trivial
  mappend x y = x <> y

编译器抱怨：

file: 'file:///d%3A/haskell/chapter15/src/Exercises.hs'
severity: 'Error'
message: 'Ambiguous occurrence `<>'
It could refer to either `Data.Semigroup.<>',
                         imported from `Data.Semigroup' at src\Exercises.hs:3:1-21
                      or `Data.Monoid.<>',
                         imported from `Data.Monoid' at src\Exercises.hs:4:1-18'
at: '14,19'
source: ''

如何解决问题？

最佳答案

通常情况下，

import Data.Monoid hiding ((<>))

（或者根本不导入Data.Monoid –正如Alec所说，Monoid类本身已经从Prelude导出了。）
那么x <> y的含义就明确了，因为x Data.Semigroup.<> y版本不在范围之内。

或者，您可以导入合格的模块之一，例如

import qualified Data.Semigroup as SG
import Data.Monoid
import Test.QuickCheck

data Trivial = Trivial deriving (Eq, Show)

instance SG.Semigroup Trivial where
  _ <> _ = Trivial

instance Monoid Trivial where
  mempty = Trivial
  mappend x y = x SG.<> y

关于haskell - 指半群或Monoid，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/44805103/