haskell - 模棱两可的情况

Question

我目前正在学习如何编写类型类。我似乎无法编写出现不明确编译错误的 Ord 类型类。

module Practice where

class  (Eq a) => Ord a  where
    compare              :: a -> a -> Ordering
    (<), (<=), (>=), (>) :: a -> a -> Bool
    max, min             :: a -> a -> a

    -- Minimal complete definition:
    --      (<=) or compare
    -- Using compare can be more efficient for complex types.
    compare x y
         | x == y    =  EQ
         | x <= y    =  LT
         | otherwise =  GT

    x <= y           =  compare x y /= GT
    x <  y           =  compare x y == LT
    x >= y           =  compare x y /= LT
    x >  y           =  compare x y == GT

    -- note that (min x y, max x y) = (x,y) or (y,x)
    max x y 
         | x <= y    =  y
         | otherwise =  x
    min x y
         | x <= y    =  x
         | otherwise =  y

错误是

Practice.hs:26:14:
    Ambiguous occurrence `<='
    It could refer to either `Practice.<=', defined at Practice.hs:5:10
                          or `Prelude.<=',
                             imported from `Prelude' at Practice.hs:1:8-15
...

等等。我认为它与 Prelude 定义的版本冲突。

Answer 1

问题在于您的函数名称与 Prelude 中的标准名称发生冲突。

要解决此问题，您可以添加显式导入声明来隐藏冲突的名称:

module Practice where

import Prelude hiding (Ord, compare, (<), (<=), (>=), (>), max, min)

...