您如何迭代列表（也许一个）

问题是这种Just launch情况需要导致a，List (Html msg)但是代码会导致返回不同的类型。

使用时List.map (\x -> x)，它本质上是无操作的。您正在遍历a List (Maybe Launch)并返回相同的东西。我建议创建另一个带有Maybe Launch值的函数并将其用作映射函数。例如：

displayLaunch : Maybe Launch -> Html Msg
displayLaunch launch =
    case launch of
        Nothing -> text "No launch"
        Just l -> text (Debug.toString l)

Just launch ->
    launch
        |> List.map displayLaunch
        |> ul []

The 2nd branch is:

    Html Msg

But all the previous branches result in:

    List (Html msg)

The 2nd branch is:

    Html Msg

But all the previous branches result in:

    List (Html msg)