Data.Time questions

[Ashley Yakeley]

Bjorn Buckwalter wrote:
> I did already create a new type for TT. However, what you are saying
> makes sense too (I've been wondering about the 'AbsoluteTime' name).
> I'm not sure that how to best handle parsing and formatting of TT
> without another type though... I'll give it some thought.

> Hmm, I realize I might also have to reconsider my 'ParseTime' instance
> for 'AbsoluteTime' before I submit that patch...

I see the problem now. TT and TAI are the same up to isomorphism, but 
they both can be written in YMD HMS format, with a 32.184s difference. 
So how to parse?

So far I note the interesting property that any given String logically 
corresponds to at most one type:

   "2008-05-01": Day
   "05:45": TimeOfDay
   "UTC-7": TimeZone
   "2008-05-01 05:45": LocalTime
   "2008-05-01 05:45 UTC-7": ZonedTime

I'm not sure if that really matters, especially as one would need a new 
union type make use of it, but we could do an "instance ParseTime 
AbsoluteTime" if we insist on virtual time-zone strings:

   "2008-01-01 00:00:00 TAI": AbsoluteTime
   "2008-01-01 00:00:32.184 TT": AbsoluteTime
   "2007-12-31 23:59:41 GPS": AbsoluteTime

I'm not sure what to do about FormatTime, though.

Another approach is to consider the TAI, TT, GPS time-scales as a 
parallel system of "zones". So we currently have

   data ZonedTime = ZonedTime LocalTime TimeZone
   utcToZonedTime :: TimeZone -> UTCTime -> ZonedTime
   zonedTimeToUTC :: ZonedTime -> UTCTime

We could add something like

   data ScaledTime = ScaledTime LocalTime TimeScale
   absoluteToScaledTime :: TimeScale -> AbsoluteTime -> ScaledTime
   scaledTimeToAbsolute :: ScaledTime -> AbsoluteTime

This would then give us

   "2008-01-01 00:00:00 TAI": ScaledTime
   "2008-01-01 00:00:32.184 TT": ScaledTime
   "2007-12-31 23:59:41 GPS": ScaledTime

This would belong in the "specialist" module Data.Time.Clock.TAI, which 
isn't included in Data.Time, so it shouldn't bother ordinary users of time.

-- 
Ashley Yakeley