A little Haskell: epoch timestamp

A need of getting the current UNIX time is something that comes up every now and then. Just this week I needed it in order to add a k8s liveness probe¹.

While it's often rather straight forward to get the Unix time as an integer in other languages², in Haskell there's a bit of type tetris involved.

1. getPOSIXTime gives me a POSIXTime, which is an alias for NominalDiffTime.
2. NominalDiffTime implements RealFrac and can thus be converted to anything implementing Integral (I wanted it as Int64).
3. NominalDiffTime also implements Num, so if the timestamp needs better precision than seconds it's easy to do (I needed milliseconds).

The combination of the above is something like

truncate <$> getPOSIXTime

In my case the full function of writing the timestamp to a file looks like this

writeTimestampFile :: MonadIO m => Path Abs File -> m ()
writeTimestampFile afn = liftIO $ do
    truncate @_ @Int64 . (* 1000) <$> getPOSIXTime >>= writeFile (fromAbsFile afn) . show