榆树 - 更新列表中的元素 [英] Elm - update elements in a list

问题描述

我刚开始在Elm编程并且遇到了困难：

I just started programming in Elm and am stuck at something:

我想有一个方法可以在某个索引处更新列表中元素的字段。

I would like to have a method that can update fields of elements in a list at a certain index.

我的签名如下所示：

updateElement : List (ID, Task) -> Int -> List (ID, Task)

包含：

type alias Task =
  { description : String, focus : Bool}

在这种情况下，我想将给定为true的索引的任务的布尔（焦点）和列表中的所有其他任务设置为false。

In this case I would like to set the boolean (focus) of the task at the index given to true and all the others tasks in the list to false.

我已经尝试过使用Elm中的数组但是我必须使用Maybe并且不认为这是一个很好的解决方案。

I already tried with arrays in Elm but then I have to work with Maybe and don't think that is a good solution.

我想我会必须使用'map'来更改列表中的元素，但我不知道如何在特定索引处更改它。

I suppose I will have to work with 'map' to change elements in my list but I don't have any clue how I could change it at a particular index.

谢谢！

推荐答案

现在您已经澄清了您的问题，真正的答案是Chad发布的两个更新的组合

Now that you've clarified your question, the real answer is a combination of the two updates Chad posted

updateElement : List (ID, Task) -> Int -> List (ID, Task)
updateElement list indexToFocusOn =
  let
    toggle index (id, task) =
      if index == indexToFocusOn then
        (id, { task | focus = true })
      else
        (id, { task | focus = false })
  in
    List.indexedMap toggle list

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