Question

首先，我对Haskell完全不熟悉，所以提前对这个问题感到抱歉，因为它可能看起来很简单，但我仍然收到此错误消息：

Couldn't match expected type `[a]' with actual type `a'
  `a' is a rigid type variable bound by
      the type signature for middle :: [a] -> a
      at myFile.lhs:18:12
Relevant bindings include
  xs :: [a] (bound at myFile.lhs:20:12)
  x :: a (bound at myFile.lhs:20:10)
  middle :: [a] -> a
    (bound at myFile.lhs:19:2)
In the first argument of `(!!)', namely `x'
In the first argument of `div', namely `x !! length xs'

失败，模块加载：无。

尝试加载时：

>middle :: [a] -> a
>middle [] = []
>middle (x:xs) = if (l `mod` 2 == 0) then xs !! (l`div` 2) - 1 else x !! l `div` 2
    where l = length xs

如果某些内容含糊不清，请发表评论。

修改由于使用 div ，我得到：

Error:     No instance for (Integral a) arising from a use of `div'
Possible fix:
  add (Integral a) to the context of
    the type signature for middle :: [a] -> a
In the expression: xs !! l `div` 2
In the expression:
  if (l `mod` 2 == 0) then xs !! (l `div` 2) - 1 else xs !! l `div` 2
In an equation for `middle':
    middle (x : xs)
      = if (l `mod` 2 == 0) then
            xs !! (l `div` 2) - 1
        else
            xs !! l `div` 2
      where
          l = length xs

Answer 1

请注意，x只是一个元素，而不是列表。因此，使用x !! anything是一种类型错误。你的意思是xs !! anything吗？

此外，

middle [] = []

是错误的，因为您必须返回一个元素，而不是一个列表。由于没有中间元素，我们只能返回底部，例如

middle [] = error "middle: empty list"

以上使得该函数成为部分函数，即如果在较大的程序中使用空列表调用该函数，则程序将崩溃。

如果您想禁止，可以将类型更改为Maybe：

middle :: [a] -> Maybe a
middle []     = Nothing
middle (x:xs) = Just (.....)

在Haskell中获取列表的中间元素时出错

1 个答案: