Add the haskell book

Haskell-book/10/Db.hs

@@ -0,0 +1,52 @@
+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..]