Question

我尝试从haskellbook解决练习并创建以下模块：

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，你只是

import Data.Monoid hiding ((<>))

（或者根本就没有导入Data.Monoid - 正如Alec评论的那样，Monoid类本身已经从Prelude导出了。）然后明确x <> y表示x Data.Semigroup.<> y，因为Data.Monoid版本不在范围内。

或者，您可以导入其中一个合格的模块，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

参考Semigroup或Monoid

1 个答案: