Daan Leijen daan at cs.uu.nl
Mon Jan 24 05:52:53 EST 2005

Keith Wansbrough wrote:
>>Rather than talk about specific Haskell code, I'd like to discuss the 
>>measurement of time in general, how these concepts might be expressed in 
>>Haskell, and what people might want from a time library. Rather than 
>>propose anything, I want to try to ask some of the right questions.
> Wow, thanks for the impressive essay!

I second that -- it was a pleasant read and a very nice
summary. Considering all the issues, it seems important to be pragmatic
about the Time library:

The first attempt at the Time library should probably focus on the 
basics that we clearly understand: TAI and UTC, and maybe time zones. 
The more tricky stuff, like calendars, and "next year" can than be dealt 
with later, on top of the well understood libraries.

-- Daan.

> I think we should also consider some of the things people might want
> to do with these times.  In particular, people often want to compute
> with time differences: next week, next month, next year, etc.  These
> are calendar operations, and defining precisely what they mean in edge
> cases is often tricky.  Even discounting leap seconds and the like,
> "next month" is sometimes 28 days into the future, sometimes 29,
> sometimes 30, and sometimes 31 - and sometimes makes little sense:
> when is "31 January 2005 plus one month"?  Similarly for "next year".
> And the decisions will be different for different applications - "plus
> one year" when computing a birthday or anniversary will be different
> from "plus one year" when computing interest on a loan.
> Any "time difference" type must deal carefully with these issues, and
> must document its behaviour on the edge cases.
> --KW 8-)
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