Clock provides the current time, as a pure alternative to:
Clock works with an F monadic context that can suspend
side effects (e.g. IO).
This is NOT a type class, as it does not have the coherence
Returns a monotonic clock measurement, if supported by the
This is the pure equivalent of Java's System.nanoTime,
or of CLOCK_MONOTONIC from Linux's clock_gettime().
The returned value can have nanoseconds resolution and represents
the number of time units elapsed since some fixed but arbitrary
origin time. Usually this is the Unix epoch, but that's not
a guarantee, as due to the limits of Long this will overflow in
the future (263 is about 292 years in nanoseconds) and the
implementation reserves the right to change the origin.
The return value should not be considered related to wall-clock
time, the primary use-case being to take time measurements and
compute differences between such values, for example in order to
measure the time it took to execute a task.
As a matter of implementation detail, the default Clock[IO]
implementation uses System.nanoTime and the JVM will use
CLOCK_MONOTONIC when available, instead of CLOCK_REALTIME
(see clock_gettime() on Linux) and it is up to the underlying
platform to implement it correctly.
And be warned, there are platforms that don't have a correct
implementation of CLOCK_MONOTONIC. For example at the moment of
writing there is no standard way for such a clock on top of
The JVM tries to do the right thing and at worst the resolution
and behavior will be that of System.currentTimeMillis.
The recommendation is to use this monotonic clock when doing
measurements of execution time, or if you value monotonically
increasing values more than a correspondence to wall-time, or
otherwise prefer realTime.
Returns the current time, as a Unix timestamp (number of time units
since the Unix epoch), suspended in F[_].
This is the pure equivalent of Java's System.currentTimeMillis,
or of CLOCK_REALTIME from Linux's clock_gettime().
The provided TimeUnit determines the time unit of the output,
its precision, but not necessarily its resolution, which is
implementation dependent. For example this will return the number
of milliseconds since the epoch:
N.B. the resolution is limited by the underlying implementation
and by the underlying CPU and OS. If the implementation uses
System.currentTimeMillis, then it can't have a better
resolution than 1 millisecond, plus depending on underlying
runtime (e.g. Node.js) it might return multiples of 10
milliseconds or more.
See monotonic, for fetching a monotonic value that
may be better suited for doing time measurements.