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如何在表链上等待第一个到达的结果?

elm-architecture elm task

2

sof · 技术社区 · 7 年前

我们可以用 Promise.race 等待第一个到达的结果 thenable Task 模块似乎还不支持它, Task.sequence Promise.all .

不可能的演示:

import Process
import Task


init () =
    ( Nothing, Cmd.batch [ after 2 "2nd", after 1 "1st" ] )


after seconds name =
    Process.sleep (1000 * seconds)
        |> Task.map (always name)
        |> Task.perform Done


type Msg
    = Done String


update (Done name) model =
    case model of
        Nothing ->
            ( Debug.log name <| Just name, Cmd.none )

        _ ->
            ( Debug.log name model, Cmd.none )


main =  
    Platform.worker
        { init = init
        , update = update
        , subscriptions = always Sub.none
        }

1st: Just "1st"
2nd: Just "1st"

1 回复 | 直到 7 年前

1

4

glennsl Namudon'tdie 7 年前

Promise.race

Maybe 要跟踪我们是否收到响应:

type Thing =
    ...

getThings : String -> Task Never (List Thing)
getThings url =
    ...


type alias Model =
    { things : Maybe (List Thing) }

type Msg
    = GotThings (List Thing)


init =
    ( { things = Nothing }
    , Cmd.batch 
          [ Task.perform GotThings (getThings "https://a-server.com/things")
          , Task.perform GotThings (getThings "https://a-different-server.com/things")
          ]
    )


update msg model =
    case msg of
        GotThings things ->
            case model.things of
                Nothing ->
                    ( { things = Just things }, Cmd.none )

                Just _ ->
                    -- if we have already received the things, ignore any subsequent requests
                    ( model, Cmd.none )


view model =
    ...