如何在榆树中将Maybe（List）转换为Just List？

Question

我有一个由字符串列表组成的列表，声明为

type alias Model =
    {board : List (List String)
    }

init : Model
init =
    {board = 
         [ ["","",""]
          ,["","",""]
         ,["","",""]
           ]
     }

在我的更新功能中，我试图通过]访问列表的内部元素>

 (getAt col (getAt row model.board))
尽管尝试编译它时收到以下错误。如何从Maybe List转换为Just List或简单地List？ 

The 2nd argument to `getAt` is not what I expect:

65|                         (getAt col (getAt row model.board))
                                        ^^^^^^^^^^^^^^^^^^^^^
This `getAt` call produces:

    Maybe (List String.String)

But `getAt` needs the 2nd argument to be:

    List a

我有一个由字符串列表组成的列表，声明为类型别名Model = {board：List（列表字符串）} init：Model init = {board = [[“”，“”，“”]， [“”，“”，...

Answer 1

首先回答您的问题：使用case .. of或case .. of。