Elm : How to get YESTERDAY date in String Date - date

Question

Let's say I have this Model:
type alias Model =
{ currentDate : String
, yesterdayDate : String
}
The CurrentDate I got from Html input type date (Date Picker) is in format YYYY-MM-DD
Html Form
input [ name "date", type_ "date", onInput UpdateDate ] []
Update.elm
UpdateDate date ->
let
-- Get Yesterday Date function here
in
( { model | currentDate = date, yesterdayDate = "" }, Cmd.none )
In this situation , how can i get yesterday Date in String ?
My idea is parse the day into INT and using subtraction method to get Yesterday day but I cannot find any way to do it... Any help is appreciate.

Answer 1

Convert the string date to Posix, convert the Posix to milliseconds since epoch, subtract the amount of milliseconds in a day, convert the resulting milliseconds back to Posix and the Posix to an ISO8601 string. Take the first 10 characters from that string.
module Main exposing (main)
import Browser
import Html exposing (Html, button, div, text)
import Html.Events exposing (onClick)
import Iso8601
import Time exposing (Posix)
sampleDate =
"2020-05-01"
subtractDays : Int -> Posix -> Posix
subtractDays days time =
(Time.posixToMillis time - (days * 24 * 60 * 60 * 1000))
|> Time.millisToPosix
subtractDaysFromIsoDate : Int -> String -> String
subtractDaysFromIsoDate days date =
Iso8601.toTime date
|> Result.map (subtractDays days >> Iso8601.fromTime >> String.left 10)
|> Result.withDefault date
main =
text <| subtractDaysFromIsoDate 1 sampleDate
Note that in this implementation if the string is not a valid date it will just be returned unmodified rather than fail. You might want to capture that this operation can fail.

Answer 2

As you can trust that you get a valid string format from html and are aware of the date package, you can split the date string into 3 strings, convert each into an integer and then construct today and yesterday as a Date value.
Questions you should ask yourself:
Do you really want to store the date as a String? The Date type might be more useful if you want to do something else then just display the string value.
And do you really want to store both today and yesterday? The latter can be easily computed when needed.
Example for string splitting:
case
String.split "-" date
|> List.map String.toInt
of
[ Just year, Just monthInt, Just day ] ->
-- convert monthInt to `Month`
-- construct current date
-- add -1 `Day`
Debug.todo "todo" 2
_ ->
Debug.todo "invalid date format" date