展平列表列表 [英] Flatten a list of lists

问题描述

我必须编写一个函数来展平列表列表.

I have to write a function that flattens a list of lists.

例如 flatten [] = [] 或 flatten [1,2,3,4] = [1,2,3,4] 或 展平 [[1,2],[3],4,5]] = [1,2,3,4,5]

我无法根据提供给 flatten 函数的内容来匹配类型.

I'm having trouble with the being able to match the type depending on what is given to the flatten function.

这是我所拥有的:

data A a = B a | C [a] deriving (Show, Eq, Ord)

flatten::(Show a, Eq a, Ord a)=>A a -> A a
flatten (C []) = (C [])
flatten (C (x:xs) ) = (C flatten x) ++ (C flatten xs)
flatten (B a) = (C [a])

据我所知，这个问题是 ++ 操作符期待它的两个参数的列表，我试图给它一些 A.我添加了 A 类型，因此该函数可以获取单个元素或元素列表.

From what I can tell the issue is that the ++ operator is expecting a list for both of its arguments and I'm trying to give it something of type A. I've added the A type so the function can either get a single element or a list of elements.

有谁知道以不同的方式执行此操作的不同方法，或解释我可以做些什么来修复类型错误?

Does anyone know a different way to do this differently, or explain what I can do to fix the type error?

推荐答案

有点不清楚你在问什么，但是展平一个列表是一个标准函数，叫做 concat in prelude with类型签名 [[a]] ->[a].

It's a bit unclear what you are asking for, but flattening a list of list is a standard function called concat in the prelude with type signature [[a]] -> [a].

如果您像上面开始的那样创建嵌套列表的数据类型，也许您想将数据类型调整为如下所示:

If you make a data type of nested lists as you have started above, maybe you want to adjust your data type to something like this:

 data Lists a = List [a] | ListOfLists [Lists a]

然后你可以将这些扁平化为一个列表；

Then you can flatten these to a list;

 flatten :: Lists a -> [a]
 flatten (List xs) = xs
 flatten (ListOfLists xss) = concatMap flatten xss

作为测试，

 > flatten (ListOfLists [List [1,2],List [3],ListOfLists [List [4],List[5]]])
 [1,2,3,4,5]

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