very rudimentary iCalendar format support in elm

ICal.elm

module ICal exposing
    ( VAlarmAction(..)
    , VAlarmTrigger(..)
    , VCalendar(..)
    , VEvent(..)
    , VEventProp(..)
    , VEventStatusValue(..)
    , eventPropString
    , eventString
    , string
    )

{-| Build VCalendar values and generate string out of it

Refer to <https://www.ietf.org/rfc/rfc2445.txt>
<https://www.kanzaki.com/docs/ical/>

Example:

    let
        cal =
            VCalendar
                [ VEvent
                    [ VEventUID "unique123"
                    , VEventSummary "Title"
                    , VEventDescription "Lorem ipsum"
                    , VEventLocation "Conference Room - F123, Bldg. 002"
                    , VEventDTStart startTime
                    , VEventDTEnd endTime
                    , VEventURL "https://www.kanzaki.com/docs/ical/"
                    , VEventStatus VEventStatusConfirmed
                    , VAlarm (VAlarmTrigger "-PT1H") "Remember the milk!" VAlarmActionDisplay
                    ]
                ]
    in
    ICal.string cal

-}

import DateFormat
import Time


type VCalendar
    = VCalendar (List VEvent)


type VEvent
    = VEvent (List VEventProp)


type VEventProp
    = VEventUID String
    | VEventSummary String
    | VEventDescription String
    | VEventLocation String
    | VEventDTStart Time.Posix
    | VEventDTEnd Time.Posix
    | VEventURL String
    | VEventStatus VEventStatusValue
    | VAlarm VAlarmTrigger String VAlarmAction


type VEventStatusValue
    = VEventStatusTentative
    | VEventStatusConfirmed
    | VEventStatusCancelled


type VAlarmTrigger
    = VAlarmTrigger String


type VAlarmAction
    = VAlarmActionAudio
    | VAlarmActionDisplay
    | VAlarmActionEmail
    | VAlarmActionProcedure


string : VCalendar -> String
string (VCalendar events) =
    String.join "\n" ("BEGIN:VCALENDAR" :: List.map eventString events) ++ "\nEND:VCALENDAR"


eventString : VEvent -> String
eventString (VEvent props) =
    String.join "\n" ("BEGIN:VEVENT" :: List.map eventPropString props) ++ "\nEND:VEVENT"


eventPropString : VEventProp -> String
eventPropString prop =
    case prop of
        VEventUID s ->
            "UID:" ++ s

        VEventSummary s ->
            "SUMMARY:" ++ s

        VEventDescription s ->
            "DESCRIPTION:" ++ s

        VEventLocation s ->
            "LOCATION:" ++ s

        VEventDTStart posixTime ->
            "DTSTART:" ++ dtString posixTime

        VEventDTEnd posixTime ->
            "DTEND:" ++ dtString posixTime

        VEventURL s ->
            "URL:" ++ s

        VEventStatus s ->
            "STATUS:" ++ statusString s

        VAlarm (VAlarmTrigger ts) s action ->
            String.join "\n"
                [ "BEGIN:VALARM"
                , "TRIGGER:" ++ ts
                , "DESCRIPTION:" ++ s
                , "ACTION:" ++ actionString action
                , "END:VALARM"
                ]



--


dtString : Time.Posix -> String
dtString posixTime =
    DateFormat.format
        [ DateFormat.yearNumber
        , DateFormat.monthFixed
        , DateFormat.dayOfMonthFixed
        , DateFormat.text "T"
        , DateFormat.hourMilitaryFixed
        , DateFormat.minuteFixed
        , DateFormat.text "00Z" --forcing `00` seconds instead of DateFormat.secondFixed
        ]
        Time.utc
        posixTime


statusString : VEventStatusValue -> String
statusString status =
    case status of
        VEventStatusTentative ->
            "TENTATIVE"

        VEventStatusConfirmed ->
            "CONFIRMED"

        VEventStatusCancelled ->
            "CANCELLED"


actionString : VAlarmAction -> String
actionString action =
    case action of
        VAlarmActionAudio ->
            "AUDIO"

        VAlarmActionDisplay ->
            "DISPLAY"

        VAlarmActionEmail ->
            "EMAIL"

        VAlarmActionProcedure ->
            "PROCEDURE"