Page 367 of HFFP

Page367.hs

module Page367 where

import Data.Time

data DatabaseItem = DbString String
                  | DbNumber Integer
                  | DbDate UTCTime
                  deriving (Eq, Ord, Show)

theDatabase :: [DatabaseItem]
theDatabase =
  [ DbDate (UTCTime
     (fromGregorian 1911 5 1)
     (secondsToDiffTime 34123))
   , DbNumber 9001
   , DbString "Hello, world!"
   , DbDate (UTCTime (fromGregorian 1921 5 1) (secondsToDiffTime 34123))
  ]

getTime :: DatabaseItem -> Maybe UTCTime
getTime (DbDate t) = Just t
getTime _ = Nothing

filterDbDate :: [DatabaseItem] -> [Maybe UTCTime]
filterDbDate x = [ getTime d | d <- x ]