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Question

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？

Answer 1

通常，你会

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。）因此， x <> y表示x Data.Semigroup.<> y Data.Monoid ，因为Data.Monoid版本不在范围内。

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