在Haskell中处理元组列表 [英] Working with list of tuples in Haskell

问题描述

我有一个Haskell中的10元组列表，我想从元组列表中获得第n个元组。但正如我所看到的，只有长度函数适用于该列表。 头，尾或 !! 功能不起作用。你能告诉我该怎么办？元组由整数和字符串组成。
例如，当我尝试这样做时：

$ $ p $ $ $ c $尾巴[（3,5，String1，String2， STRING3， 串，4， STRING5， String6， String7， String8），（3,5， 字符串1， String2的， STRING3， 串，4， STRING5， String6， String7， String8），（3,5， 字符串1， String2的， STRING3， 串，4， STRING5， String6， String7， String8） ]

我从拥抱中收到此错误消息：

 错误 - 无法找到
 ***的show函数表达式：tail [（3,5，String1，String2，String3， 串，4， STRING5， String6， String7， String8），（3,5， 字符串1， String2的， STRING3， 串，4， STRING5， String6， String7，String8），（3,5，String1，String2，String3，String4，String5，String6，String7，String8）] 
 ***类型：[（Integer，Integer，[Char]，[Char]，[Char]，[Char]，[Char]，[Char]，[Char]，[Char]）] 
  




解决方案

以下是如何声明显示 3元组实例。希望这可以说明这个想法，您可以将它扩展到更多元素：



 
 import Data.List（插入）
 
实例（显示a，显示b，显示c）=>显示（a，b，c）其中
显示（a，b，c）=（++（intercalate，（[show a ，show b，show c]））++）
 

您可以像读取逻辑一样读取实例声明：如果我可以显示类型a，b和c的值，那么然后，我可以显示类型为（a，b，c）的元组，并且以下是方法。

GHC为一个15元组的所有元素定义一个Show实例，所以你可能不需要在你的情况下定义它。

I have a list of 10-tuples in Haskell and I want to get nth tuple from that list of tuples. But as I saw, only length function worked with that list. head, tail or !! functions didn't work. Can you tell me what should I do? The tuples are composed of integers and strings. For example when I try this :

tail [(3,5,"String1","String2","String3","String4","String5","String6","String7","String8"),(3,5,"String1","String2","String3","String4","String5","String6","String7","String8"),(3,5,"String1","String2","String3","String4","String5","String6","String7","String8")]

I get this error message from hugs:

ERROR - Cannot find "show" function for:
*** Expression : tail [(3,5,"String1","String2","String3","String4","String5","String6","String7","String8"),(3,5,"String1","String2","String3","String4","String5","String6","String7","String8"),(3,5,"String1","String2","String3","String4","String5","String6","String7","String8")]
*** Of type    : [(Integer,Integer,[Char],[Char],[Char],[Char],[Char],[Char],[Char],[Char])]

解决方案

Here's how to declare a Show instance for a 3-tuple. Hopefully this illustrates the idea and you can extend it to more elements:

import Data.List (intercalate)

instance (Show a, Show b, Show c) => Show (a, b, c) where
  show (a, b, c) = "(" ++ (intercalate "," ([show a, show b, show c])) ++ ")"

You can read the instance declaration just like logical implication: if I can show values of type a, b, and c, then I can show a tuple of type (a, b, c), and here's how.

GHC defines a Show instance for everything up to a 15-tuple, so you probably won't need to define this in your case.

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