榆木-将Msg转换为Cmd Msg [英] Elm - Turn Msg into Cmd Msg

问题描述

我正在尝试从 elm-lang教程修改一个简单的应用程序首先更新模型，然后触发另一个更新.

I'm trying to modify a simple app from the elm-lang tutorial to first update the model, then trigger another update.

update msg model =
  case msg of
    MorePlease ->
      (model, getRandomGif model.topic)

    NewGif (Ok newUrl) ->
      ( { model | gifUrl = newUrl }, Cmd.none)

    NewGif (Err _) ->
      (model, Cmd.none)

    -- my addition
    NewTopic newTopic ->
      ({ model | topic = newTopic}, MorePlease)

这在编译器中失败，因为NewTopic分支:

This fails in the compiler because the NewTopic branch:

The 3rd branch has this type:

( { gifUrl : String, topic : String }, Cmd Msg )

But the 4th is:

( { gifUrl : String, topic : String }, Msg )

因此，我的消息需要输入Cmd Msg.如何将我的消息"转换为Cmd消息?

So my Msg needs to be type Cmd Msg. How can I turn" my Msg into a Cmd Msg?

注意:我认识到有一种更简单的方法来进行此更改，但我想从根本上理解Elm

推荐答案

实际上没有必要将 Msg 转换为 Cmd Msg .请记住， update 只是一个函数，因此您可以递归调用它.

There is really no need to turn Msg into a Cmd Msg. Remember that update is just a function, so you can call it recursively.

您的 NewTopic 案例处理程序可以简化为:

Your NewTopic case handler can be simplified to this:

NewTopic newTopic ->
    update MorePlease { model | topic = newTopic}

如果您确实希望Elm Architecture在这种情况下触发Cmd，则可以将 Cmd.none 的简单 map 映射到所需的消息:

If you really truly wanted the Elm Architecture to fire off a Cmd for this scenario, you could do a simple map of Cmd.none to your desired Msg:

NewTopic newTopic ->
    ({ model | topic = newTopic}, Cmd.map (always MorePlease) Cmd.none)

(实际上不推荐)

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