package async_extra

Overview

A Schedule.t describes a (potentially repeating) schedule by selecting a subset of all seconds using the set operations in t. For example:

• every 5 min after the hour :

  Mins [ 5 ]

• 9am to 10am every day :

  Between (Time.Ofday.create ~hr:9 (), Time.Ofday.create ~hr:10 ())

• Every weekday at 3pm:

  And
          [ Weekdays [ Mon; Tue; Wed; Thu; Fri ]
          ; At [ Time.Ofday.create ~hr:15 () ] ]

• On the 15th of every month at midnight:

  And
          [ Days [ 15 ]
          ; At [ Time.Ofday.start_of_day ] ]

• 9:30 am on weekends, and 5 am on weekdays

  Or
          [ And
              [ Weekdays [ Sat; Sun ]
              ; At [ Time.Ofday.create ~hr:9 ~min:30 () ] ]
          ; And
              [ Weekdays [ Mon; Tue; Wed; Thu; Fri ]
              ; At [ Time.Ofday.create ~hr:5 () ] ]
          ]

Zones and Tags

On top of this selection language there are two labeling branches of the variant that are important.

In_zone (zone, t) expresses that all of t should be evaluated relative to the time zone given.

Tag (tag, t) tags anything matching t with tag.

Combining these we can express something complex like the on-call groups across three offices:

let weekdays         = Weekdays Day_of_week.weekdays in
let working_hours    = Between Time.Ofday.((create ~hr:8 (), create ~hr:18 ())) in
let working_schedule = And [ weekdays; working_hours ] in
let offices =
  let (!!) = Time.Zone.find_exn in
  Location.Abbrev.([
    tot, !!"America/New_York"
  ; hkg, !!"Asia/Hong_Kong"
  ; ldn, !!"Europe/London" ])
in
List.map offices ~f:(fun (office, zone) ->
  In_zone (zone, Tag (office, working_schedule)))

after which we can use the tags function to extract the groups on call at any moment.

Daylight Savings Time

Schedules are expressed in terms of wall clock time, and as such have interesting behavior around daylight savings time boundaries. There are two circumstances that might affect a schedule. The first is a repeated time, which occurs when time jumps back (e.g. 2:30 may happen twice in one day). The second is a skipped time, which occurs when time jumps forward by an hour.

In both cases Schedule does the naive thing. If the time happens twice and is included in the schedule it is included twice. If it never happens Schedule makes no special attempt to artificially include it.

Interface

• In_zone: see the discussion under Zones and Tags above
• Tag: see the discussion under Zones and Tags above
• And, Or, Not: correspond to the set operations intersection, union, and complement.
• If_then_else (A, B, C): corresponds to (A && B) || (NOT A && C), useful for dealing with schedules that change during certain times of the year (holidays, etc.)
• At: the exact times given on every day

• Shift: shifts an entire schedule forward or backwards by a known span. For example:

  • Shift ((sec 3.), Secs[10]) = Secs [13]
  • Shift ((sec (-3.)), Secs[10]) = Secs [7]

• Between: the contiguous range between the start and end times given. The schedule can span the midnight boundary. If the start and end ofdays are the same then:

  | Exclusive, Exclusive  -> Not At
        | Inclusive, Inclusive -> At
        | Exclusive, Inclusive
        | Inclusive, Exclusive -> Always

• Zoned_between: As Between, but allows expressing the start and end times in different zones
• Secs: the given seconds of a minute, repeated every minute
• Mins: the given minutes of an hour, repeated every hour
• Hours: the given hours of a day, repeated every day
• Weekdays: the given weekdays, repeated every week
• Days: the given calendar dates, repeated every month (if the day occurs in that month)
• Weeks: the given weeks (numbered by ISO 8601), repeated every year
• Months: the given months, repeated every year
• On: the exact dates given
• Before: all seconds before the given boundary (inclusive or exclusive)
• After: all seconds after the given boundary (inclusive or exclusive)
• Always: the set of all seconds
• Never: the empty set

'a indicates whether the schedule currently has an established zone.

'b is the type of the tag used in this schedule. In many cases it can be unspecified. See tags for more.

Items that take int lists silently ignore ints outside of the viable range. E.g. Days [32] will never occur.

Return a sequence of schedule changes over time that will never end.

If your schedules ends, you will continue to receive `No_change_until_at_least with increasing times forever.

The return type indicates whether includes t start_time is true and delivers a sequence of subsequent changes over time.

The times returned by the sequence are strictly increasing and never less than start_time. That is, `No_change_until_at_least x can never be followed by `Enter x, only by (at least) `Enter (x + 1s).

if emit is set to Transitions_and_tag_changes then all changes in tags will be present in the resulting sequence. Otherwise only the tags in effect when a schedule is entered are available.

The `In_range | `Out_of_range flag in `No_change_until_at_least indicates whether the covered range is entirely within, or outside of the time covered by the schedule and is only there to help with bookkeeping for the caller. `In_range | `Out_of_range will never disagree with what could be inferred from the `Enter and `Leave events.

The sequence takes care to do only a small amount of work between each element, so that pulling the next element of the sequence is always cheap. This is the primary motivation behind including `No_change_until_at_least.

The Time.t returned by `No_change_until_at_least is guaranteed to be a reasonable amount of time in the future (at least 1 hour).