Question

我在Haskell中有一个10元组列表，我希望从元组列表中获得第n个元组。但正如我所看到的，只有长度功能与该列表一起使用。头，尾或 !! 功能无效。你能告诉我该怎么办？元组由整数和字符串组成。例如，当我尝试这个时：

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")]

我从拥抱中收到此错误消息：

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])]

Answer 1

以下是如何为3元组声明Show实例。希望这说明了这个想法，你可以将它扩展到更多元素：

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])) ++ ")"

您可以像逻辑含义一样阅读实例声明： if 我可以显示类型a，b和c的值，然后我可以显示类型的元组（a，b，c），这是如何。

GHC为一个15元组的所有内容定义了一个Show实例，所以你可能不需要在你的情况下定义它。

使用Haskell中的元组列表

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