[Haskell-cafe] Re: why doesn't time allow diff of localtimes ?

[Bayley, Alistair]

> briand at aracnet.com
> 
> Maciej Piechotka <uzytkownik2 at gmail.com> wrote:
> 
> > On Wed, 2010-03-31 at 19:29 -0700, briand at aracnet.com wrote:
> > > wagnerdm at seas.upenn.edu wrote:
> > > 
> > > > Two values of LocalTime may well be computed with respect to
> > > > different timezones, which makes the operation you ask for
> > > > dangerous. First convert to UTCTime (with localTimeToUTC), then
> > > > compare.
> > > 
> > > that makes sense.  unfortunately getting the current timezone to
> > > convert to UTC results in the dreaded IO contamination problem...
> > > 
> > > Brian
> > 
> > Hmm. Where do you get the local times from in the first place?
> > 
> 
> read it from a file of course :-)
> 
> I think I've got it figured out.  it's not too ugly.
> 
> One interesting "hole" in the system is that buildTime can return a
> LocalTime _or_ a UTCTime.  That means the same string used to
> generate a time can give you two different times.
> 
> It seems as thought it should be restricted to always returning a
> UTCTime.  If it's going to return a local time it should require an
> extra argument of a timezone, shouldn't it ?


LocalTime is just day+time, sans TimeZone (so it could be in any
timezone). ZonedTime is LocalTime with TimeZone.

The functions to convert to UTCTime give a clue:

zonedTimeToUTC :: ZonedTime  -> UTCTime
localTimeToUTC :: TimeZone  -> LocalTime  -> UTCTime

i.e. ZonedTime can convert with no additional information (so in a sense
is equivalent), but for LocalTime you need to say which TimeZone to use.

If all you have in your file is day_time, you're going to have to get a
TimeZone from somewhere, in order to convert to UTCTime. If all you're
doing is diffing the LocalTimes, and they are all in the same TimeZone,
then any TimeZone will do, I guess.

Perhaps add what you learn to one or more of these pages:
  http://haskell.org/haskellwiki/Cookbook/Dates_And_Time
  http://haskell.org/haskellwiki/Libraries_and_tools/Time_library
or create a new wiki page with better docs for Data.Time?

(aside: do we have a policy for naming pages that augment library
documentation?
what about under here?
  http://haskell.org/haskellwiki/Library
e.g.
  http://haskell.org/haskellwiki/Library/Time
)

Alistair
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