Re: [boost] [chrono/date] Unit specifiers arithmetic

Subject: Re: [boost] [chrono/date] Unit specifiers arithmetic
From: Howard Hinnant (howard.hinnant_at_[hidden])
Date: 2013-05-06 10:10:16

On May 6, 2013, at 9:55 AM, "Vicente J. Botet Escriba" <vicente.botet_at_[hidden]> wrote:

> Le 06/05/13 15:34, Howard Hinnant a écrit :
>> On May 6, 2013, at 6:31 AM, "Vicente J. Botet Escriba" <vicente.botet_at_[hidden]> wrote:
>> 
>>> Hi,
>>> 
>>> Moving from checked dates to unchecked ones by default and forcing to user the Unit specifiers year/month/day in any date constructor (even the unchecked ones) has some ugly consequences. Note that I was able to build an unchecked date as
>>> 
>>> date d(2013,5,6, no_check);
>>> 
>>> Given
>>> 
>>>  year y;
>>> month m;
>>>  day d;
>>> 
>>> the following will not compile now as the constructors expects a year but y+1 is an int.
>>> 
>>>  date d(y+1, m, d);
>>> 
>>> The user needs to type
>>> 
>>>  date d(year(y+1), m, d);
>>> 
>>> Is this what we want to provide to the user or should we add basic arithmetic on these unit specifiers?
>> This is an example of where flexible ordering and implicit last unit starts to shine.  Examples from my 2011 paper:
>> 
>>         start = mon <= jan4/(d.year()-1);
>> 
>>         date next_start = mon <= jan4/(start.year()+1);
>> 
>> Or translated to your example and syntax:
>> 
>>     date d(d, m, y+1);  // works because last unit can be int

> Last unit or any unit but only one?

Any two explicit units should disambiguate.  We could even take it more lax than that and still avoid ambiguity if desired.

A warning though:  We want to make the checked syntax more attractive than the unchecked syntax, else all dates will be unnecessarily unchecked in practice.

>> 
>> If we start adding arithmetic to the unit specifier day, it becomes indistinguishable from a duration.  Now it is possible that we could use durations as unit specifiers.
> -1

Agreed.

Howard