How get yesterday in Haskell? For the current day - of course it works like this:
date :: IO (Integer,Int,Int) -- :: (year,month,day)
date = getCurrentTime >>= return . toGregorian . utctDay
But for yesterday? Does that work with diffUTCTime?
With sql:
select current date, current date - 1 day from sysdba.routine
08.11.2018 07.11.2018
But with Haskell?
You can use the addUTCTime :: NominalDiffTime -> UTCTime -> UTCTime function with as difference -nominalDay. For example:
Prelude Data.Time.Clock> fmap (addUTCTime (-nominalDay)) getCurrentTime
2018-11-07 15:27:57.8510597 UTC
or when you want to obtain the Gregorian date:
Prelude Data.Time.Clock Data.Time.Calendar> fmap (toGregorian . utctDay . addUTCTime (-nominalDay)) getCurrentTime
(2018,11,7)