Data.Thyme.Docs

thyme tries to maintain API compatibility with time and each module exports the same functions as corresponding one in time. There are however some differences to keep in mind:

• UTCTime is an opaque type and there is no UTCTime constructor. Since GHC 7.8 you can use the UTCTime pattern synonym. In any case, utcTime can convert it to a UTCView.

• UniversalTime similarly has the ModJulianDate pattern synonym, along with modJulianDate.

• Years are Ints, not Integers: you may need fromIntegral.

• If you'd rather not use lens or vector-space directly, most modules list functions under a ‘Compatibility’ section that mimics the time API.

• Where a third party library uses time, you can use toThyme and fromThyme from Data.Thyme.Time.Core to convert between the corresponding types.

• Data.Thyme.Time provides Num, Real, Fractional, and RealFrac instances for DiffTime and NominalDiffTime. Avoid using this in new code, because of reasons.

Any other differences not covered here may be unintentional. Please check the list of issues on GitHub and file a new one if necessary.

One of the issues arising from Num, Real, Fractional, and RealFrac instances for e.g. NominalDiffTime is that we must also supply nonsensical operations such as (*): multiplying two quantities of dimension T gives something of dimension T², yet according to the type of (*) the result is another NominalDiffTime of dimension T.

A more principled approach is to invoke our geometric intuition and think of instants in time as points in a one-dimensional affine space—that is, a line without an origin per se; a time-line, if you will. Taking the distance (displacement) between two points in this space amounts to finding the (signed) duration between two instants in time. These durations are akin to vectors in the geometric sense, and as such we can add, subtract, or scaled them by some factor to obtain another duration. Conversely a point plus (or minus) a vector results in another point, and likewise an instant in time plus (or minus) a duration takes us to another instant in time.

The AffineSpace and VectorSpace classes from vector-space allows us to express exactly this idea: UTCTime correspond to points on the time-line, and is therefore an instance of AffineSpace with the associated

type Diff UTCTime = NominalDiffTime

as the type of durations between instants. It may help to think of the provided operations as having these restricted types:

(.-.) :: UTCTime -> UTCTime -> NominalDiffTime
(.+^) :: UTCTime -> NominalDiffTime -> UTCTime
(.-^) :: UTCTime -> NominalDiffTime -> UTCTime

In turn NominalDiffTime is an instance of VectorSpace (and also AdditiveGroup) which comes with the following operations:

(^+^) :: NominalDiffTime -> NominalDiffTime -> NominalDiffTime
(^-^) :: NominalDiffTime -> NominalDiffTime -> NominalDiffTime

Said instance also defines type Scalar NominalDiffTime = Rational, so we can scale NominalDiffTimes thusly:

(*^) :: Rational -> NominalDiffTime -> NominalDiffTime
(^*) :: NominalDiffTime -> Rational -> NominalDiffTime
(^/) :: NominalDiffTime -> Rational -> NominalDiffTime

On operator naming: remember that . are points, and ^ are vectors. Thus (.+^) corresponds to the operation point plus vector, and (^-^) means vector minus vector, and so on.

Similarly, AbsoluteTime is an AffineSpace, and DiffTime is a VectorSpace, with

type Diff AbsoluteTime = DiffTime
type Scalar DiffTime = Rational

If you must have Num, Real, Fractional, and RealFrac for DiffTime and NominalDiffTime, they are provided as orhpan instances in the Data.Thyme.Time module.

Since a large part of thyme's functionality is involved in converting back and forth between various representations of time, it's convenient to expose these as Control.Lens.Isos from the lens package, and field accessors as Control.Lens.Lenses.

A full lens tutorial is however beyond the scope of this documentation. As far as using thyme is concerned, it's sufficient to restrict ourselves to a subset of the lens API:

view   :: Lens' s a -> s -> a
review :: Iso'  s a -> a -> s
over   :: Lens' s a -> (a -> a) -> s -> s

There are infix versions of the above:

(^.) = flip view
(#)  = review
(%~) = over

An Iso' can be used wherever a Lens' is required, so it is helpful to think of view and over as also having the following types:

view   :: Iso' s a -> s -> a
over   :: Iso' s a -> (a -> a) -> s -> s

You can use a Lens' s a to extract an a out of an s with view. An Iso s a' means you can also construct an s from an a with review. Furthermore Lens's and Iso's can be chained with the standard (.) function composition.

(.) :: Lens' s u -> Lens' u a -> Lens' s a
(.) :: Iso'  s u -> Iso'  u a -> Iso'  s a

Composing an Iso' with a Lens' however ‘downgrades’ the result to a Lens'.

The tutorial below has some examples.

UTCTime from Data.Time.Clock has unbounded range, whereas UTCTime from Data.Thyme.Clock has a usable range just under 600,000 years, between the following times inclusive:

-290419-11-07 19:59:05.224192 UTC
 294135-11-26 04:00:54.775807 UTC

thyme is therefore not Y294K-compliant.

UTCTime, NominalDiffTime, DiffTime, and so on from Data.Thyme.Clock have resolutions of 1 μs = 10^-6 s, while those from Data.Time.Clock have resolutions of 1 ps = 10^-12 s.

However keep in mind that a call to gettimeofday(3) or clock_gettime(3) takes on the order of ~0.4μs on a relatively recent (circa 2016) desktop, so the coarser resolution is not really an issue in almost all applications.

Data.Thyme.Clock.UTCTime is implemented as newtype wrappers around a single Int64, so that addUTCTime and diffUTCTime both compile down to a single arithmetic operation. Likewise Data.Thyme.Clock.DiffTime (and NominalDiffTime) are also newtypes over Int64s.

Data.Time.Clock.UTCTime on the other hand comprises of two Integers internally: one for the Day and another for the DiffTime time-of-day. Here diffUTCTime has to account for the difference in Days, while addUTCTime must normalise its result to ensure utctDayTime stays within T.posixDayLength.

Furthermore, any Data.Time.Clock.DiffTime (and NominalDiffTime) longer than ~2.15 ms exceeds 2^31 ps, and would overflow a native GHC.Prim.Int# on 32-bit architectures, incurring the cost of using BigNats inside Integer. Even for 64-bit Int#s, 2^63 ps corresponds to only ~106 days, and it's not inconceivable to deal with durations longer than this. In both cases, there is the additional overhead of working with lifted Integers.