On 29-nov-2007, at 14:44, Simon Marlow wrote:
Brandon S. Allbery KF8NH wrote:
On Nov 19, 2007, at 16:06 , Arthur van Leeuwen wrote:
here is a puzzle for you: try converting a
System.Posix.Types.EpochTime into either a
System.Time.CalendarTime or a Data.Time.Clock.UTCTime without
going through
read . show or a similar detour through strings.
fromEnum and/or toEnum are helpful for this kind of thing, and I
am occasionally tempted to bind "cast = toEnum . fromEnum" because
I need it so much.
Let's ignore System.Time since it's obsoleted by Data.Time.
Yes. That's what I wanted to do at first. :)
I just spent a little while trying to solve this puzzle, and it
turns out there *is* a right way to do this: for t :: EpochTime,
posixSecondsToUTCTime (fromRational (toRational t) :: POSIXTime)
Which, unfortunately, does not work with time-1.1.1 which is in GHC
6.6.1.
Going through Rational is the right solution, though. My hackish
detour was to use fromIntegral . toInteger . fromEnum
You want to go via Data.Time.Clock.POSIXTime, because that's what
an EpochTime is. Now, EpochTime does not have an Integral
instance, because it is the same as C's time_t type, which is not
guaranteed to be an integral type. You have fromEnum, but that
would be a hack: there's no guarantee that EpochTime fits in an
Int, and if EpochTime is a fractional value you lose information.
But you *can* convert to a Rational with toRational, and from there
you can get to a POSIXTime with fromRational (also realToFrac would
do).
It turns out there are good reasons for all this, it just ends up
being quite obscure. I'll put the above formula in the docs.
Yes, that would be good! Note that both the docs for DiffTime and
EpochTime state that
they contain seconds, and both are somewhat unclear as to whether
they contain any
higher precision than whole seconds.
With kind regards, Arthur.
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