在列表的每个顶层元素周围包装[] [英] wraps [] around each top-level element of list

问题描述

  wrap [1,2,3]应该输出[[1]，[2]，[3]] 
 wrap [[1]，[2]，[ 3]]应输出[[[1]]，[[2]]，[[3]]] 
  

我的实现是：

  wrap [] = [] 
 wrap（x：xs）= [x ]：[wrap xs] 
  

和haskell输出错误：发生检查：无法构造无限类型：预期类型：[t] - > [t]
实际类型：[t] - > [[t]]

p>

解决方案 包装 wrap xs 。你不想包装整个结果。您只想包装列表中其余部分的每个单独元素，这正是 wrap xs 已经做过的事情。因此， wrap 是：

  wrap :: [a]  - > ; [[a]] 
 wrap [] = [] 
 wrap（x：xs）= [x]：wrap xs 
  

错误告诉你，你正在尝试使用 [t] ，其中GHC期望 t 。这正是我们上面所说的，你要包装太多次。

wrap [1,2,3] should output [[1],[2],[3]]
wrap [[1],[2],[3]] should output [ [[1]], [[2]], [[3]] ]

my implementation is:

wrap [] = []
wrap (x:xs) = [x] :  [wrap xs]

and haskell output an error: Occurs check: cannot construct the infinite type: t ~ [t]

Expected type: [t] -> [t] Actual type: [t] -> [[t]]

解决方案

[wrap xs] wraps the entire result of wrap xs. You don't want to wrap the entire result. You only want to wrap each individual element of the remainder of the list and that is exactly what wrap xs already does. Thus, wrap is:

wrap :: [a] -> [[a]]
wrap [] = []
wrap (x:xs) = [x] : wrap xs

The error is telling you that you are trying to use a [t] where GHC expects a t. This is exactly what we said above, that you are wrapping something too many times.

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