How do you get a millisecond-precision unix timestamp in Haskell?

Question

What is the Haskell equivalent of this shell command?

$ date "+%s%3N"
1489694389603

Answer 1

Using the time package:

1. Use getPOSIXTime, which returns a NominalDiffTime. This represents a fractional number of seconds, with a precision of 10^-12 seconds.

2. Multiply 1000 to convert from seconds to milliseconds.

3. NominalDiffTime implements the RealFrac class, so you can use round to convert to an integer.

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import Data.Time.Clock.POSIX (getPOSIXTime)

λ> (round . (* 1000)) <$> getPOSIXTime
1489694668011