Elm没有正确地推断出Nothing值的类型

Question

我有一段Elm代码(getProjectView函数为简洁而省略):

type Model = Maybe List Project

model : Model
model = Nothing

getView : Model -> Html any
getView model =
  case model of
    Just projects ->
      ul [] (List.map getProjectView projects)

    Nothing -> p [] [ text "Still loading..." ]

当我尝试编译以下代码段时,编译器失败并出现以下错误:

-- TYPE MISMATCH --------- E:\dev\irrelephant-code\client\elm\Views\Projects.elm

Tag `Maybe.Just` is causing problems in this pattern match.

32|     Just projects ->
        ^^^^^^^^^^^^^
The pattern matches things of type:

    Maybe a

But the values it will actually be trying to match are:

    Model

表明编译器不能推断出这Nothing是一个类型的值Model(它又是类型的别名Maybe List Project).

我在这做错了什么？有没有办法明确地将此Nothing标记为Model类型的值？

我使用的是elm v0.18.0

Answer 1

要定义模型作为type alias的Maybe (List Product).现在,使用type关键字,您将定义一个带有一个值的新union/tag类型Maybe,它需要类型List和的参数Product.

一个工作示例是https://ellie-app.com/3MhPcgGzXqRa1/0