Core.Data.Clock

The standard package for working with dates and times in Haskell, time, is awkward. That's a subjective judgment, but over the years there have been few areas more frustrating than trying to do pragmatic things with calendars and clocks. This module represents some opinionated approaches to working with times and dates, and a place to collect some hard-won idioms for converting between things.

Our original use was wanting to conveniently measure things happening on distributed computer systems. Since machine clock cycles are in units of nanoseconds, this has the nice property that, assuming the system clock is not corrupted, two subsequent events from the same source process are likely to have monotonically increasing timestamps. And even if the system clock goes to hell, they're still decently likely to be unique per device. Make for good keys.

So the timestamp type herein Time is nanoseconds since the Unix epoch; which in (signed) 64 bits means that you can represent times between early in the morning of 21 September 1677 through just before midnight on 11 April 2262. The primary use isn't doing calendaring, though; it's just working with machine generated timestamps in distributed systems and for conveying start and end times around in your program.

There are quite a few other time formats around the Haskell ecosystem. You can use the fromTime and intoTime methods of the Instant typeclass to convert from one to another if you need to.

Time type

data Time

Number of nanoseconds since the Unix epoch.

The Show and Externalize instances display the Time as seconds with the nanosecond precision expressed as a decimal amount after the interger, ie:

>>> t <- getCurrentTimeNanoseconds
>>> formatExternal t
"2014-07-31T23:09:35.274387031Z"

However this doesn't change the fact the underlying representation counts nanoseconds since epoch:

>>> show $ unTime t
"1406848175274387031"

There is a Externalize instance that is reasonably accommodating:

>>> parseExternal "2014-07-31T13:05:04.942089001Z" :: Maybe Time
Just 2014-07-31T13:05:04.942089001Z

>>> parseExternal "1406811904.942089001" :: Maybe Time
Just 2014-07-31T13:05:04.942089001Z

>>> parseExternal "1406811904" :: Maybe Time
Just 2014-07-31T13:05:04.000000000Z

In case you're wondering, the valid range of nanoseconds that fits into the underlying Int64 is:

>>> formatExternal (minBound :: Time)
"1677-09-21T00:12:43.145224192Z"

>>> formatExternal (maxBound :: Time)
"2262-04-11T23:47:16.854775807Z"

so in a quarter millenium's time, yes, you'll have the Y2262 Problem. Haskell code from today will, of course, still be running, so in the mid Twenty-Third century you will need to replace this implementation with something else.

Since: 0.3.3

Instances

Bounded Time

Defined in Core.Data.Clock

Methods

minBound :: Time

maxBound :: Time

Enum Time

Defined in Core.Data.Clock

Methods

succ :: Time -> Time

pred :: Time -> Time

toEnum :: Int -> Time

fromEnum :: Time -> Int

enumFrom :: Time -> [Time]

enumFromThen :: Time -> Time -> [Time]

enumFromTo :: Time -> Time -> [Time]

enumFromThenTo :: Time -> Time -> Time -> [Time]

Eq Time

Defined in Core.Data.Clock

Methods

(==) :: Time -> Time -> Bool

(/=) :: Time -> Time -> Bool

Ord Time

Defined in Core.Data.Clock

Methods

compare :: Time -> Time -> Ordering

(<) :: Time -> Time -> Bool

(<=) :: Time -> Time -> Bool

(>) :: Time -> Time -> Bool

(>=) :: Time -> Time -> Bool

max :: Time -> Time -> Time

min :: Time -> Time -> Time

Read Time

Defined in Core.Data.Clock

Methods

readsPrec :: Int -> ReadS Time

readList :: ReadS [Time]

readPrec :: ReadPrec Time

readListPrec :: ReadPrec [Time]

Show Time

Defined in Core.Data.Clock

Methods

showsPrec :: Int -> Time -> ShowS

show :: Time -> String

showList :: [Time] -> ShowS

Generic Time

Defined in Core.Data.Clock

Associated Types

type Rep Time :: Type -> Type

Methods

from :: Time -> Rep Time x

to :: Rep Time x -> Time

ToJSON Time

Defined in Core.Data.Clock

Methods

toJSON :: Time -> Value

toEncoding :: Time -> Encoding

toJSONList :: [Time] -> Value

toEncodingList :: [Time] -> Encoding

FromJSON Time

Defined in Core.Data.Clock

Methods

parseJSON :: Value -> Parser Time

parseJSONList :: Value -> Parser [Time]

Externalize Time

Timestamps are formatted as per ISO 8601:

2022-06-20T14:51:23.544826062Z

Defined in Core.Encoding.External

Methods

formatExternal :: Time -> Rope

parseExternal :: Rope -> Maybe Time

type Rep Time

Defined in Core.Data.Clock

type Rep Time = D1 ('MetaData "Time" "Core.Data.Clock" "core-data-0.3.4.0-IgqpNqCYboI2URbmdeDx4t" 'True) (C1 ('MetaCons "Time" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 Int64)))

getCurrentTimeNanoseconds :: IO Time

Get the current system time, expressed as a Time (which is to say, number of nanoseconds since the Unix epoch).

Since: 0.3.3

Conversions

class Instant a where

Convert between different representations of time. Our Time timestamp has nanosecond precision so converting from a type with lesser or greater precision will require you to either pad with zeros or to round to the nearest nanosecond (who the hell has picoseconds of anything anyway?) if writing an instance of this type.

Since: 0.3.3

Methods

fromTime :: Time -> a

intoTime :: a -> Time

Instances

Instant Int64

Defined in Core.Data.Clock

Methods

fromTime :: Time -> Int64

intoTime :: Int64 -> Time

Instant UTCTime

Defined in Core.Data.Clock

Methods

fromTime :: Time -> UTCTime

intoTime :: UTCTime -> Time

Instant ElapsedP

Defined in Core.Data.Clock

Methods

fromTime :: Time -> ElapsedP

intoTime :: ElapsedP -> Time

Instant POSIXTime

Defined in Core.Data.Clock

Methods

fromTime :: Time -> POSIXTime

intoTime :: POSIXTime -> Time

Internals

unTime :: Time -> Int64

If you need to manipulate the date or calculate elapsed time then you can dig out the underlying Int64 here. We have not provided instances of Num, Real, or Integral for the timestamp type because adding two timestamps doesn't really make sense. You can use intoTime to reconstruct a timestamp subsequently if necessary.

Since: 0.3.3

epochTime :: Time

The occasion of the Unix epoch, 1970-01-01T00:00:00.0Z.

Since: 0.3.3