Elm:
Обчислення дати у майбутньому або минулому

Що і чому?

Calculating a future or past date involves figuring out a date some days ahead or behind a specific date. Coders need this for features like reminders, subscription renewals, or historical data analysis.

How to:

Як це робити:

Elm makes date computations quite clear. Let’s calculate today’s date plus 10 days and minus 30 days.

import Time
import Date exposing (Date)
import Task
import Browser

type Msg
    = GotTime Posix

type alias Model =
    { today : Maybe Date
    , plusTen : Maybe Date
    , minusThirty : Maybe Date
    }

init : () -> ( Model, Cmd Msg )
init _ =
    ( { today = Nothing, plusTen = Nothing, minusThirty = Nothing }
    , Time.now |> Task.perform GotTime
    )

update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        GotTime posix ->
            let
                today = posix |> Time.toDate
                plusTen = Date.add Days 10 today
                minusThirty = Date.add Days -30 today
            in
            ( { model | today = Just today, plusTen = Just plusTen, minusThirty = Just minusThirty }
            , Cmd.none
            )

-- Add your view and subscriptions if needed, for a complete program

Sample Output:

Model
    { today = Just (Date 2023-03-25)
    , plusTen = Just (Date 2023-04-04)
    , minusThirty = Just (Date 2023-02-23)
    }

Deep Dive

Поглиблений розгляд

Elm’s handling of dates hinges on the Time and Date modules. Historically, date manipulation in programming is tricky due to time zones and leap years. Elm simplifies this by using Posix time, counting milliseconds since the Unix epoch.

Alternatives? You could use JavaScript’s Date, but it has issues around daylight saving time and mutability. Elm’s Date is immutable and handles complexities silently.

For in-depth date logic, like recurring events, you might need a custom solution. Elm’s simplicity also means some use cases require extra effort, like installing third-party date libraries.

See Also

Дивись також

• Elm’s official Time documentation
• rtfeldman/elm-iso8601-date-strings for ISO 8601 string parsing