Question

通过：

data Nested List a = Elem a | List [NestedList a]
flatten :: NestedList a -> [a]
flatten (Elem a) = [a]
flatten (List (x:xs)) = flatten x ++ flatten (List xs)
flatten (List []) = []

这失败了：

data NestedList a = Elem a | List [NestedList a]
flatten :: NestedList a -> [a]
flatten (Elem a) = [a]
flatten (List (x:xs)) = flatten x : flatten (List xs)
flatten (List []) = []

错误是：

Couldn't match expected type `a' with actual type `[a]'
  `a' is a rigid type variable bound by
      the type signature for flatten :: NestedList a -> [a]
      at 007.hs:2:12
Relevant bindings include
  xs :: [NestedList a] (bound at 007.hs:4:18)
  x :: NestedList a (bound at 007.hs:4:16)
  flatten :: NestedList a -> [a] (bound at 007.hs:3:1)
In the first argument of `(:)', namely `flatten x'
In the expression: flatten x : flatten (List xs)

差异为++而非:。我知道前者是附加的，后者是附加的，但为什么:在这种情况下不起作用？我不明白这个错误。

Answer 1

实际上++是连接运算符，它希望运算符在这种特定情况下都是一个列表。由于flatten会返回一个列表，++会很高兴地将它们连接起来。

但是，:只在列表中添加一个项目。由于flatten x中的flatten x : flatten (List xs)会返回一个列表，:会抛出此错误。

Haskell 99问题＃7：Prepend vs Append to List

1 个答案: