在Haskell中挣扎着列表 [英] Struggling with lists in Haskell

问题描述

以下是我的函数：

  create_matrix ::（Read t，Integral t）=> [字符串]  - > t  - > [[t]] 
 create_list_of_lists（x：xs）num = [read x`div`z | z < -  [1..num]]：create_list_of_lists xs num 
  

当我像这样运行它时：

  create_list_of_lists [1212，3232] 3 
  

我得到所需的输出，即

  [ [1212,606,404]，[3232,1616,1077] 
  

但错过了最后的']'最后，并向我显示这个错误：

 例外：hondt.hs：39：1-81：非穷举在函数中的模式create_list_of_lists 
  

我想它与类型有关，但我不知道我是什么'm做错了。

解决方案

发生什么事是由于Haskell的懒惰，GHCi将前两个子列表打印为它会收到它们，但会遇到错误。此错误是因为您尚未为 create_list_of_lists 定义所有可能的输入，即当第一个参数为空时。您需要

  create_list_of_lists [] num = ??? 
 create_list_of_lists（x：xs）num = [read x`div`z | z<  -  [1..num]]：create_list_of_lists xs num 
  

c $ c> [] ，你最终会调用 create_list_of_lists [] num ，这当前是未定义的。

I'm having trouble with a list of lists in Haskell.

Here's my function:

create_matrix :: (Read t, Integral t) => [String] -> t -> [[t]]
create_list_of_lists (x:xs) num = [read x `div`z | z <- [1..num]] : create_list_of_lists xs num

When i run it like this:

create_list_of_lists ["1212","3232"] 3

i get sort off the desired output, which is

[[1212,606,404],[3232,1616,1077]

but it misses the last ']' at the end, and presents me with this error:

Exception: hondt.hs:39:1-81: Non-exhaustive patterns in function create_list_of_lists

I guess it has to do with types, but i don't know what i'm doing wrong.

解决方案

What is happening is that due to Haskell's laziness GHCi is printing out the first two sub-lists as it receives them, but then runs into an error. This error is because you haven't defined all the possible inputs for create_list_of_lists, namely when the first argument is empty. You'll need

create_list_of_lists [] num = ???
create_list_of_lists (x:xs) num = [read x `div`z | z <- [1..num]] : create_list_of_lists xs num

Without the clause for [], you'll end up calling create_list_of_lists [] num, which is currently undefined.

这篇关于在Haskell中挣扎着列表的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！