我正在尝试使用https://en.wikibooks.org/wiki/Haskell/Monoids这个页面在Haskell中使用monoid。我在终端机上输入了以下信息(在导入Data.Monoid
之后):
class Monoid a where
mempty :: a
mappend :: a -> a -> a
mconcat :: [a] -> a
mconcat = foldr mappend memptyhere
newtype Sum a = Sum { getSum :: a }
instance Num a => Monoid (Sum a) where
mempty = Sum 0
Sum x `mappend` Sum y = Sum (x + y)
但是,当我尝试Sum 5 <> Sum 6 <> Sum 10
时,会收到以下消息:
<interactive>:115:1: error:
• Non type-variable argument in the constraint: Semigroup (Sum a)
(Use FlexibleContexts to permit this)
• When checking the inferred type
it :: forall a. (Semigroup (Sum a), Num a) => Sum a
我不知道这些错误是什么,以及为什么Sum 5 <> Sum 6 <> Sum 10
无法正常工作。
答案 0 :(得分:5)
问题是您正在使用自己的Sum
类型和Monoid
类型的类,且其操作符<>
与您的版本 的功能不相同mappend
中的。如果要在多行GHCi提示符下输入此程序:
> :{
Prelude| ...paste your program in here...
Prelude| :}
>
,然后尝试以下操作:
> Sum 5 `mappend` Sum 6 `mappend` Sum 7
Sum 5 `mappend` Sum 6 `mappend` Sum 7 :: Num a => Sum a
没有错误。如果您在deriving (Show)
类型中添加了Sum
,您甚至会得到所需的答案!
Ok, modules loaded: none.
λ> :{
Prelude| class Monoid a where
Prelude| mempty :: a
Prelude| mappend :: a -> a -> a
Prelude| mconcat :: [a] -> a
Prelude| mconcat = foldr mappend mempty
Prelude| newtype Sum a = Sum { getSum :: a } deriving (Show)
Prelude| instance Num a => Monoid (Sum a) where
Prelude| mempty = Sum 0
Prelude| Sum x `mappend` Sum y = Sum (x + y)
Prelude| :}
λ> Sum 5 `mappend` Sum 6 `mappend` Sum 7
Sum {getSum = 18}
λ>
在GHCi中覆盖库定义的规则可能有点复杂,因此将其放入xxx.hs
文件中并通过:l xxx.hs
加载到GHCi中进行测试可能是一个更好的主意。如果您尝试将此程序作为xxx.hs
文件加载,则有关该问题的信息会更加清晰:
MonoidExample2.hs:7:19-24: error:
Ambiguous occurrence ‘Monoid’
It could refer to
either ‘Prelude.Monoid’,
imported from ‘Prelude’ at MonoidExample2.hs:1:1
(and originally defined in ‘GHC.Base’)
or ‘Main.Monoid’, defined at MonoidExample2.hs:1:1
然后,您可以使用特殊的import Prelude
语法来隐藏不需要的库定义。以下版本可作为独立程序运行:
import Prelude hiding (Monoid, mempty, mappend, mconcat, (<>))
class Monoid a where
mempty :: a
mappend :: a -> a -> a
mconcat :: [a] -> a
mconcat = foldr mappend mempty
newtype Sum a = Sum { getSum :: a } deriving (Show)
instance Num a => Monoid (Sum a) where
mempty = Sum 0
Sum x `mappend` Sum y = Sum (x + y)
(<>) :: Monoid a => a -> a -> a
(<>) = mappend
main :: IO ()
main = print $ Sum 5 <> Sum 6 <> Sum 10