Proposal: Add parsing (and some other changes) to the time package

[Bjorn Bringert]

I propose adding parsing to the time package. I also propose some minor
other changes and bugfixes. I have implemented the changes that I  
consider
reasonably clear-cut. The rest I will implement after community input.

All already implemented changes are available from:
http://www.cs.chalmers.se/~bringert/darcs/time/

All objections, comments, etc. are welcome. I especially would like  
feedback on the
unimplemented enhancements, and any objections to the already  
implemented ones.

I have created ticket #1007 to track this proposal: http:// 
hackage.haskell.org/trac/ghc/ticket/1007

Proposal:

== Executive summary ==
(see below for more details).

=== Implemented enhancements ===

* Add time parsing to the time package, including Read instances.

* Add fromMondayStartWeek and fromSundayStartWeek.

* Add secondsToDiffTime and picosecondsToDiffTime.

=== Unimplemented enhancements ===

* Change %S to: the number of whole seconds.

* Add %q: the number of picoseconds (including trailing zeroes).

* Add %Q: decimal point and second decimals, without trailing zeros.

* Add %f: The century part of the week date year.

* Change %Z to produce the time zone offset if the time zone name is "".

* Add Eq and Ord instances for ZonedTime?

* Add Typeable and Data instances for all the time package types.

=== Fixed bugs ===

* formatTime "%c" did not include the time zone when applied to  
ZonedTime or UTCTime.

* s/propleptic/proleptic/ in some Haddock comments.

* formatTime documentation of range of %U and %W was wrong, should  
include 00.

* The showWeekDate Haddock comment was missing the example.

* The taiEpoch Haddock comment was missing the epoch time.

=== Unfixed bugs ===

* Haddock comments for toJulianYearAndDay and isJulianLeapYear talk  
about the Gregorian calendar.


== Details ==

=== Implemented enhancements ===

* Add time parsing to the time package.

Rationale:
The old System.Time has had a TODO  "* add functions to parse strings to
`CalendarTime' (some day...)" for a long time. The question about  
date parsing
comes up once in a while on the mailing lists
(e.g. http://comments.gmane.org/gmane.comp.lang.haskell.cafe/16438).

* Add fromMondayStartWeek and fromSundayStartWeek to  
Data.Time.Calendar.OrdinalDate.

Rationale:
I can't find any duals of mondayStartWeek and sundayStartWeek. They
are useful when implementing parsing for %W and %U.

* Add secondsToDiffTime and picosecondsToDiffTime.

Rationale:
As has come up on haskell-cafe
(http://comments.gmane.org/gmane.comp.lang.haskell.cafe/15653), it
takes a while to figure out how to make DiffTime values.

secondsToDiffTime is not that important since it is just another name
for fromInteger, but I suspect that it would be used a lot.  Using
fromRational to create a DiffTime from a number of picoseconds is a
bit of a hassle, so having a picosecondsToDiffTime would be useful.



=== Unimplemented enhancements ===

* formatTime: Change %S to: the number of whole seconds.
* formatTime: Add %q: the number of picoseconds (including trailing  
zeroes).
* formatTime: Add %Q: decimal point and second decimals, without  
trailing zeros.
If the number of picoseconds is zero, nothing is produced (not even  
the decimal point).

Rationale:
Currently %S includes decimals if there are any. This is different
from strftime, and there is no format specifier for just the integer
part of the seconds. It would be nice to have such a specifier to
implement many standard date formats (e.g. RFC 822). Also a specifier
for second decimals would also help when using %s. Currently there is
no reasonable way to get more than integer second precision with
since-epoch timestamps.  Thus the current %S would be equivalent to
%S%Q under this proposal.


* Add %f: The century part of the week date year.

Rationale:
There is a %g specifier for the last two digits of the week date  
year, but no specifier
for the century. %C cannot be used, since the normal century and the  
week date
century can differ:
{{{
 > formatTime defaultTimeLocale "%Y %G" (fromGregorian 2000 1 1)
"2000 1999"
}}}

* Change %Z to produce the time zone offset if the time zone name is "".

Rationale:
Without this, if you format a ZonedTime which contains an unnamed  
timezone,
%Z produces the empty string. This is invalid in many formats. It  
would be better
to output the offset when there is no timezone name.


* Add Eq and Ord instances for ZonedTime?

Rationale:
It's useful at least for the QuickCheck properties, but probably also
elsewhere. There are lots of standard functions that want Eq and Ord.

Problems:

Should ZonedTimes be converted to UTC and compared, or should times in
different timezones never be equal? I would argue for the latter, but
I suspect that this problem is the reason why it never got included in
the first place.

Interestingly, there is a derived instance of Eq for TimeZone, which
means that timezones with the same offset but different names are
considered different. Following this logic, I think that ZonedTimes
with different timezones should also be different. This should be
noted in the documentation.

* Add Typeable and Data instances for all the time package types.

Rationale:
It's useful and doesn't hurt anyone.


=== Fixed bugs ===

* formatTime "%c" does not include the time zone even when applied to  
ZonedTime
or UTCTime, since "%c" is handled by the FormatTime LocalTime instance:

{{{
 > fmap (formatTime System.Locale.defaultTimeLocale "%c") getZonedTime
"Sat Nov 11 19:12:45.395568  2006"
 > fmap (formatTime System.Locale.defaultTimeLocale "%c") getCurrentTime
"Sat Nov 11 18:13:52.010944  2006"
}}}

* s/propleptic/proleptic/ in Data/Time/Calendar/JulianYearDay.hs and  
Data/Time/Calendar/OrdinalDate.hs

* formatTime documentation of range of %U and %W was wrong, should  
include 00.

* The showWeekDate haddock comment is:  "show in ISO 8601 Week Date  
format as yyyy-Www-dd (e.g."

* The taiEpoch haddock comment is just "The epoch of TAI, which is"


=== Unfixed bugs ===

* The haddock comments for toJulianYearAndDay and isJulianLeapYear in
Data.Time.Calendar.JulianYearDay talk about the proleptic Gregorian  
calendar.
This looks like a copy-and-paste problem. What is the calendar  
called? Proleptic Julian?


/Björn Bringert