Question

我正在尝试使用unfoldr在Haskell中实现选择排序，但我的函数不断抛出类型错误，我不确定如何解决它。

我的选择代码排序：

unfoldSort = unfoldr select
select [] = Nothing
select xs = Just (m, delete m xs)
   where m = minimum xs

这是我收到的错误消息：

• Ambiguous type variable ‘a0’ arising from a use of ‘select’
  prevents the constraint ‘(Ord a0)’ from being solved.
  Relevant bindings include
    unfoldSort :: [a0] -> [a0]
  Probable fix: use a type annotation to specify what ‘a0’ should be.
  These potential instances exist:
    instance (Ord b, Ord a) => Ord (Either a b)
      -- Defined in ‘Data.Either’
    instance Ord Ordering -- Defined in ‘GHC.Classes’
    instance Ord Integer
      -- Defined in ‘integer-gmp-1.0.0.1:GHC.Integer.Type’
    ...plus 23 others
    ...plus 38 instances involving out-of-scope types
    (use -fprint-potential-instances to see them all)
• In the first argument of ‘unfoldr’, namely ‘select’
  In the expression: unfoldr select
  In an equation for ‘unfoldSort’: unfoldSort = unfoldr select

Answer 1

当您说：{/ 1>}时，unfoldSort会返回Maybe。

unfoldSort :: t -> Maybe (t, t)

但这是真的吗？请注意，unfoldSort 的返回类型必须与unfoldr的返回类型相同，因为您只是返回它给您的任何内容。

unfoldr有签名：

unfoldr :: (b -> Maybe (a, b)) -> b -> [a]

请注意，它会返回一个列表，而不是Maybe，这意味着您的unfoldSort也必须返回一个列表。

更改签名，使其返回列表，而不是Maybe。

选择使用展开式在Haskell中排序

1 个答案: