怎么昨天在Haskell？当前日期 - 1天？

Question

怎么昨天在Haskell？对于当天 - 当然它的工作原理如下:

date :: IO (Integer,Int,Int) -- :: (year,month,day)
date = getCurrentTime >>= return . toGregorian . utctDay

但对于昨天？这适用于diffUTCTime吗？

用sql:

select current date, current date - 1 day from sysdba.routine 
08.11.2018 07.11.2018

但是对于Haskell？

Answer 1

您可以使用该addUTCTime :: NominalDiffTime -> UTCTime -> UTCTime功能作为差异-nominalDay.例如:

Prelude Data.Time.Clock> fmap (addUTCTime (-nominalDay)) getCurrentTime
2018-11-07 15:27:57.8510597 UTC

或者当您想要获得格里高利日期时:

Prelude Data.Time.Clock Data.Time.Calendar> fmap (toGregorian . utctDay . addUTCTime (-nominalDay)) getCurrentTime
(2018,11,7)