_.hs

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))
    ]

-- 1. Write a function that filters for DbDate values and returns a list
-- of the UTCTime values inside them.

filterDbDate :: [DatabaseItem] -> [UTCTime]
filterDbDate = foldr foo []
  where
    foo db acc = case db of
        DbDate x -> x : acc
        _ -> acc

-- 2. Write a function that filters for DbNumber values and returns a list
-- of the Integer values inside them.
filterDbNumber :: [DatabaseItem] -> [Integer]
filterDbNumber = foldr foo []
  where
    foo db acc = case db of
        DbNumber x -> x : acc
        _ -> acc

-- 3. Write a function that gets the most recent date.
mostRecent :: [DatabaseItem] -> UTCTime
mostRecent = maximum . foldr foo []
  where
    foo db acc = case db of
        DbDate x -> x : acc
        _ -> acc

mostRecent2 :: [DatabaseItem] -> UTCTime
mostRecent2 = foldr foo utcEmpty
  where
    utcEmpty = UTCTime (fromGregorian 0 1 1) (secondsToDiffTime 0)
    foo db acc = case db of
        DbDate x -> max x acc
        _ -> acc

-- 4. Write a function that sums all of the DbNumber values.
sumDb :: [DatabaseItem] -> Integer
sumDb = foldr foo 0
  where
    foo db acc = case db of
        DbNumber x -> x + acc
        _ -> acc

-- 5. Write a function that gets the average of the DbNumber values.
-- You'll probably need to use fromIntegral
-- to get from Integer to Double.
avgDb :: [DatabaseItem] -> Double
avgDb = (\(s,n) -> fromIntegral s / n) . foldr foo (0,0)
  where
    foo db (acc,n) = case db of
        DbNumber x -> (x + acc, 1 + n)
        _ -> (acc,n)