[m-rev.] calendar module proposal

[Ian MacLarty]

On Tue, Jan 27, 2009 at 5:01 PM, Paul Bone <pbone at csse.unimelb.edu.au> wrote:
> On Tue, Jan 27, 2009 at 04:02:11PM +1100, Ian MacLarty wrote:
>> Hi,
>>
>> Here's a proposal for a calendar module for the standard library.
>> Could interested people take a look and give me some feedback?
>> Since this module only implements the Gregorian calendar, it might not
>> be suitable for working with dates prior to 1582.  Perhaps it should be
>> renamed to gregorian_calendar or gcalendar?
>
> I'd prefer gregorian_calendar.
>

Yes, I prefer that too.

>> Index: library/calendar.m
>> ===================================================================
>> RCS file: library/calendar.m
>> diff -N library/calendar.m
>> --- /dev/null 1 Jan 1970 00:00:00 -0000
>> +++ library/calendar.m        27 Jan 2009 03:51:25 -0000
>> @@ -0,0 +1,961 @@
>> +%-----------------------------------------------------------------------------%
>> +% vim: ft=mercury ts=4 sw=4 et wm=0 tw=0
>> +%-----------------------------------------------------------------------------%
>> +% Copyright (C) 2009 The University of Melbourne.
>> +% This file may only be copied under the terms of the GNU Library General
>> +% Public License - see the file COPYING.LIB in the Mercury distribution.
>> +%-----------------------------------------------------------------------------%
>> +%
>> +% File: calendar.m.
>> +% Main authors: maclarty
>> +% Stability: low.
>> +%
>> +% Proleptic Gregorian calendar utilities.
>> +%
>> +%-----------------------------------------------------------------------------%
>> +%-----------------------------------------------------------------------------%
>> +
>> +:- module calendar.
>> +:- interface.
>> +
>> +:- import_module io.
>> +
>> +%-----------------------------------------------------------------------------%
>> +
>> +    % A point on the Proleptic Gregorian calendar, to the nearest second.
>> +    %
>> +:- type date.
>> +
>> +    % A period of time measured in years, months, days, hours, minutes and
>> +    % seconds.
>> +    %
>> +:- type duration.
>> +
>> +:- type month
>> +    --->    january
>> +    ;       february
>> +    ;       march
>> +    ;       april
>> +    ;       may
>> +    ;       june
>> +    ;       july
>> +    ;       august
>> +    ;       september
>> +    ;       october
>> +    ;       november
>> +    ;       december.
>> +
>> +
>> +    % Date components.
>> +    %
>> +:- type year == int.         % Year 0 is 1 BC, -1 is 2 BC, etc.
>> +:- type day_of_month == int. % 1..31 depending on the month and year
>> +:- type hour == int.         % 0..23
>> +:- type minute == int.       % 0..59
>> +:- type second == int.       % 0..61 (60 and 61 are for leap seconds)
>
> Should microseconds be supported?  Even if some system libraries don't
> allow access to the absolute time in mincroseconds it might still be
> useful to be able to use them.
>

Okay, I'll add support for that.

>> +
>> +:- type day_of_week
>> +    --->    sunday
>> +    ;       monday
>> +    ;       tuesday
>> +    ;       wednesday
>> +    ;       thursday
>> +    ;       friday
>> +    ;       saturday.
>
> IMHO a week starts on Monday, so sunday should sort after monday (after
> saturday), but I'm not sure what is 'official'.
>

I'll change it to monday.

>
>> +%----------------------------------------------------------------------------%
>> +
>> +:- type date
>> +    --->    date(
>> +                dt_year         :: int,
>> +                dt_month        :: int,
>> +                dt_day          :: int,
>> +                dt_hour         :: int,
>> +                dt_minute       :: int,
>> +                dt_second       :: int
>> +            ).
>> +
>> +:- type duration
>> +    --->    duration(
>> +                dur_years   :: int,
>> +                dur_months  :: int,
>> +                dur_days    :: int,
>> +                dur_hours   :: int,
>> +                dur_minutes :: int,
>> +                dur_seconds :: int
>> +            ).
>> +
>
> The best representation of duration I've ever seen is Postgreql's
> internal INTERVAL structure.  It represents durations in terms of just
> three values (the minimum possible number of values): months, days, and
> seconds, although it allows a user to use any unit to express the
> interval.
>
> I recommend taking a look at Postgresql's INTERVAL:
>
> http://www.postgresql.org/docs/8.3/interactive/datatype-datetime.html#AEN4976
>

Okay, thanks. I'll take a look and submit a revised module shortly.

At some point I'd also like to add some functions for custom
formatting of dates.  Do you have any suggestions?

Ian.
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