How do I get the current time in Elm?

问题

I'm running elm-repl to play around with the language.

I'd like to see what the current time is. How would I do that? It doesn't appear to be possible with the current library. Why is that?

EDIT: I made a package to help with this. http://package.elm-lang.org/packages/z5h/time-app

This was asked around elm 0.15 - things are different in elm 0.17 & 0.18: see How do I get the current time in Elm 0.17/0.18?

回答1:

Update for 0.19 It is not possible to get the current time using the standard library.. You need to use elm/time. As with 0.18, all you need is a command and Msg to handle the result

type Msg
    = OnTime Time.Posix 

getTime : Cmd Msg
getTime = 
    Task.perform OnTime Time.now

Update for 0.18 This has got simpler again. Now all you need is a command and Msg to handle the result

type Msg
    = OnTime Time 

getTime : Cmd Msg
getTime = 
    Task.perform OnTime Time.now

See this Ellie

Original answer

With 0.17, this got a whole lot easier. There is now a Task in the Time library. So for example, we now have:

Time.now
|> Task.Perform NoOp CurrentTime

回答2:

You can use the Time package and/or the Date package.

Here's a contrived example which uses both:

import Signal
import Time exposing (every, second)
import Date exposing (year, hour, minute, second, fromTime)
import Graphics.Element exposing (show)

main =
  Signal.map currentTime (Time.every Time.second)

currentTime t =
  let date' = fromTime t
      hour' = toString (Date.hour date')
      minute' = toString (Date.minute date')
      second' = toString (Date.second date')
      year' = toString (year date')
      now = "The current time is: " ++ hour' ++ ":" ++ minute' ++ ":" ++ second'
  in 
      show now

回答3:

To resolve my own question, I've created a variant of StartApp that includes a timestamp on each action.
So the update function has signature:
update : action -> Time -> model -> (model, Effects action)

The Gist is here. https://gist.github.com/z5h/41ca436679591b6c3e51

回答4:

Elm 0.19

https://elm-lang.org/examples/time
https://elm-lang.org/examples/clock
time https://ellie-app.com/3f6X2DW4cbma1
Analog svg clock https://ellie-app.com/3dYXjr3bJNWa1

Below I set inital time as unix time start Time.millisToPosix 0, but you can set it to Nothing and later to Just time or pass it with Flag.

module Main exposing (main)

import Browser
import Html exposing (Html)
import Task
import Time exposing (Posix)


main : Program () Model Msg
main =
    Browser.element
        { init = \_ -> init
        , view = view
        , update = update
        , subscriptions = \_ -> Sub.none
        }



-- MODEL


type alias Model =
    { zone : Time.Zone
    , now : Posix
    }


init : ( Model, Cmd Msg )
init =
    ( Model Time.utc (Time.millisToPosix 0), Task.perform Zone Time.here )



-- UPDATE


type Msg
    = Zone Time.Zone
    | Now Posix


update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        Zone zone ->
            ( { model | zone = zone }, Task.perform Now Time.now )

        Now now ->
            ( { model | now = now }, Cmd.none )



-- VIEW


formatTime zone posix =
    (String.padLeft 2 '0' <| String.fromInt <| Time.toHour zone posix)
        ++ ":"
        ++ (String.padLeft 2 '0' <| String.fromInt <| Time.toMinute zone posix)
        ++ ":"
        ++ (String.padLeft 2 '0' <| String.fromInt <| Time.toSecond zone posix)


view : Model -> Html Msg
view model =
    Html.div []
        [ Html.text <| formatTime model.zone model.now
        ]

回答5:

If you want the time as of program start you can do the following:

Now.elm

module Now where

import Native.Now

loadTime : Float
loadTime = Native.Now.loadTime

Native/Now.js

Elm.Native.Now = {};

Elm.Native.Now.make = function(localRuntime) {

  localRuntime.Native = localRuntime.Native || {};


  localRuntime.Native.Now = localRuntime.Native.Now || {};

  if (localRuntime.Native.Now.values) {
    return localRuntime.Native.Now.values;
  }

  var Result = Elm.Result.make(localRuntime);

  return localRuntime.Native.Now.values = {
    loadTime: (new window.Date).getTime()
  };

};

your code

programStart = Now.loadTime

回答6:

You can see pdoherty926's answer for how to do something with the current time in Elm.

elm-repl doesn't have the ability to work with Signals, and time "changes over time" so it's a signal. There also isn't a Task for getting the time, that I know of. Nor a way to execute tasks in the repl, though I expect that will be feature in the future.

回答7:

There are two main ways I can think of to work with the current time in Elm:

Write / use a native module to make a function that returns the current time in ms (or returns a task that will do the same). This method isn't generally recommended. I think #2 is a better approach. But an example of #1 can be found here: https://github.com/evancz/task-tutorial/blob/1.0.2/src/TaskTutorial.elm (see the getCurrentTime function)
Write a program using the Elm application architecture (https://github.com/evancz/elm-architecture-tutorial/), and then feed the current time signal as an input to the update cycle, updating the model with the new current time at every interval of your choosing. Then all of your other methods can just fetch the current time synchronously off of the model.

回答8:

The answer (for 0.18) from Simon H got me started in the right direction, but I did have some trouble working out how to actually do something with that time. (user2167582 adds a comment to Simon's answer which asks the same thing: how do you 'get the time out?').

My specific problem was that I wanted to include the current time in the body of a POST to the server.

I eventually solved that and was quite pleased with the end result -- the use of Task.andThen meant that I in my postTime function I can just use timestamp as a 'regular' float-valued parameter (which it is when the task gets run, I suppose).

My full SO answer is here.

Below is the solution I came up with and here it is in Ellie:

module Main exposing (..)

import Html exposing (..)
import Html.Events exposing (..)
import Http
import Json.Decode as JD
import Json.Encode as JE
import Task
import Time


type alias Model =
    { url : String
    }


type Msg
    = PostTimeToServer
    | PostDone (Result Http.Error String)


update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        PostTimeToServer ->
            ( model, postTimeToServer model.url )

        PostDone _ ->
            ( model, Cmd.none )


view : Model -> Html Msg
view model =
    div []
        [ div []
            [ button [ onClick PostTimeToServer ] [ Html.text "POST the current time." ]
            ]
        ]


postTimeToServer : String -> Cmd Msg
postTimeToServer url =
    let
        getTime =
            Time.now

        postTime t =
            JD.string
                |> Http.post url (JE.float t |> Http.jsonBody)
                |> Http.toTask

        request =
            getTime                                            <<-- Here is
                |> Task.andThen postTime                       <<-- the key bit.
    in
        Task.attempt PostDone request


main =
    Html.program
        { init = ( Model "url_here", Cmd.none )
        , update = update
        , view = view
        , subscriptions = always Sub.none
        }

来源：https://stackoverflow.com/questions/29453679/how-do-i-get-the-current-time-in-elm

标签

time

elm