haskell-函数练习

total_money :: Double -> Double -> Double -> Double
total_money a p y = let x = (1 + p) in a * x ** y

back_money total p n = let p1 = 1 + p in total * p1**n * ( 1 - p1) / ( 1 - p1**n )

abs :: Integral a => a -> a

abs n | n > 0 = n
      | otherwise = -n

month' :: Int -> Int
month' 1 = 31
month' 2 = 28
month' 3 = 31
month' 4 = 30
month' 5 = 31
month' 6 = 30
month' 7 = 31
month' 8 = 31
month' 9 = 30
month' 10 = 31
month' 11 = 30
month' 12 = 31
month' _  = error "invalid month"