Time Libraries Rough Draft

[Ashley Yakeley]

This seems to be roughly where we're at with the time libraries. It's 
now three modules, Clock, LeapSeconds, and Calendar. Most people will 
have no need for LeapSeconds.

I've added two more things which haven't been discussed much. The first 
is a getCPUTime function that returns the current "CPU time". This has 
an arbitrary base, but it doesn't hiccup. The Unix interface for this is 
a "clock()" function that returns time in some unit, and 
CLOCKS_PER_SECOND, which is a constant to convert to seconds. I'm 
assuming there's a Windows equivalent to this.

The second thing is a type-synonym of Rational for Julian dates. Julian 
dates are a standard representation of UT1, where one unit is one day, 
and UT1 is a good choice for representing Earth-based times in the far 
past or future continuously.


System.Time.Clock:

  * UTC arithmetic

  * types or type-synonyms for Julian days and dates (for UT1)

  * getting the current UTC time

  * getting the current "CPU time"

System.Time.LeapSeconds:

  * TAI arithmetic

  * leap-second table type

  * conversion between UTC and TAI with table

System.Time.Calendar (largely undiscussed so far):

  * time zones

  * getting the locale time zone

  * converting times to Gregorian "calendrical" format

  * calendrical arithmetic
    e.g. one month after March 31st

  * possibly "incomplete dates"
    e.g. "May 1" for all years

  * parsing and showing dates and times

Left for other libraries (standard or user):

  * sunrise/sunset, moon phase, solstices & equinoxes, etc.

  * figuring out time zone from position on Earth

  * civil summertime zone adjustment calculation,
    interface to zoneinfo database

  * calendar systems other than Gregorian

  * generalised sets over time
    e.g. "2pm-4pm every 4th Saturday of the month"



module System.Time.Clock ( ... ) where

  -- | standard Julian count of Earth days
  type JulianDay = Integer

  newtype DiffTime = MkDiffTime Integer

  timeToSISeconds :: DiffTime -> Rational

  siSecondsToTime :: Rational -> DiffTime

  data UTCTime = MkUTCTime JulianDay DiffTime

  newtype UTCDiffTime = MkUTCDiffTime Integer

  utcTimeToUTCSeconds :: UTCDiffTime -> Rational

  utcSecondsToUTCTime :: Rational -> UTCDiffTime

  addUTCTime :: UTCDiffTime -> UTCTime -> UTCTime

  diffUTCTime :: UTCTime -> UTCTime -> UTCDiffTime

  ...more arithmetic on UTCDiffTime...

  -- | standard Julian dates for UT1, 1 = 1 day
  type JulianDate = Rational

  getCurrentTime :: IO UTCTime

  getCPUTime :: IO DiffTime


-- | most people won't need this module
module System.Time.LeapSeconds ( ... ) where

  -- | TAI
  type AbsoluteTime = MkAbsoluteTime Integer

  addAbsoluteTime :: DiffTime -> AbsoluteTime -> AbsoluteTime

  diffAbsoluteTime :: AbsoluteTime -> AbsoluteTime -> DiffTime

  -- | TAI - UTC during this day
  type LeapSecondTable = JulianDay -> Int

  utcDayLength :: LeapSecondTable -> JulianDay -> DiffTime

  utcToTAITime :: LeapSecondTable -> UTCTime -> TAITime

  taiToUTCTime :: LeapSecondTable -> TAITime -> UTCTime


module System.Time.Calendar
  ... TBD ...

-- 
Ashley Yakeley, Seattle WA