Haskell：如何附加到元组列表列表？

Question

对不起，我刚开始学习Haskell。我不明白如何将元组附加到每个子列表。

我目前的名单是：

[[("String", "String", 2.0)],[("String", "String", 2.0),("String", "String", 2.0)]]

我想在每个子列表中添加一个元组，所以我的新列表是：

[[("String", "String", 2.0),("String","String",0.0)],[("String", "String", 2.0),("String", "String", 2.0),("String","String",0.0)]]

我正在尝试编写一个函数，但它一直在给我一个错误

append :: a -> [a] -> [a]
append i (x:xs) = x : append i xs
append i [] = i : []

然后我做：

map append ("String","String",0.0)

这给了我很多错误，比如错误的“类型匹配”。我不知道如何在Haskell中附加。

Answer 1

[[("String", "String", 2.0)],[("String", "String", 2.0),("String", "String", 2.0)]]的类型是Fractional t => [[(String, String, t)]]，或简化[[(String, String, Double)]]。

因此，为了将元素附加到每个嵌套列表，我们需要有一个函数(String, String, Double) -> [[(String, String, Double)]] -> [[(String, String, Double)]]。换句话说，a -> [[a]] -> [[a]]：

append :: a -> [[a]] -> [[a]]
append x xs = map (\xss -> xss ++ [x]) xs

最后

> append ("String","String",0.0) [[("String", "String", 2.0)],[("String", "String", 2.0),("String", "String", 2.0)]]
[[("String","String",2.0),("String","String",0.0)],[("String","String",2.0),("String","String",2.0),("String","String",0.0)]]

顺便说一下，请注意在列表末尾添加元素是On），您可以考虑将其添加到开头：

append' :: a -> [[a]] -> [[a]]
append' x = map ((:) x)