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|
module Db 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))
]
filterDbDate :: [DatabaseItem] -> [UTCTime]
filterDbDate xs = foldr f1 [] (filter f2 xs)
where f1 (DbDate d) acc = d : acc
f2 (DbDate d) = True
f2 _ = False
filterDbNumber :: [DatabaseItem] -> [Integer]
filterDbNumber xs = foldr f1 [] (filter f2 xs)
where f1 (DbNumber d) acc = d : acc
f2 (DbNumber d) = True
f2 _ = False
mostRecent :: [DatabaseItem] -> UTCTime
mostRecent xs = maximum (filterDbDate xs)
sumDb :: [DatabaseItem] -> Integer
sumDb xs = foldr (+) 0 (filterDbNumber xs)
avgDb :: [DatabaseItem] -> Double
avgDb xs = (fromIntegral (sumDb xs)) / (fromIntegral (length (filterDbNumber xs)))
fibs :: [Integer]
fibs = 1 : scanl (+) 1 fibs
fibs20 :: [Integer]
fibs20 = take 20 (1 : scanl (+) 1 fibs)
fibs100 :: [Integer]
fibs100 = takeWhile (\x -> x < 100) fibs
fibsN :: Int -> Integer
fibsN x = fibs !! x
factorial :: [Integer]
factorial = scanl (*) 1 [1..]
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