SimpleTime.hs

-- Simple time module, no fuss over real world complications, just a simple way
-- to calculate sums and differenes of hours and minutes on a 12 hour
-- clock using divs and mods.

module SimpleTime where

-- Time in a simple hour:minute format
data Time = Time Int Int

-- Helper function to display times correctly
show2Digits m | (length $ show m) == 1 = "0" ++ show m
              | otherwise           = show m 

showHour h | h == 0    = "12"
           | otherwise = show h
           
instance Show Time where
    show (Time h m) = (showHour h) ++ ":" ++ (show2Digits m)

-- Safe constructor for time
infixr 9 %:
(%:) :: Int -> Int -> Maybe Time
h%:m | (h <= 12) && (h >= 0) &&
       (m <= 60) && (m >= 0) = Just (Time h m)
     | otherwise             = Nothing 

-- Unsafe constructor for time -- throws error on improper format.
infixr 9 !:
(!:) :: Int -> Int -> Time
h!:m | (h <= 12) && (h >= 0) &&
       (m <= 60) && (m >= 0) = Time h m
     | otherwise             = error "Bad time format." 

infixr 8 %+
(Time h1 m1) %+ (Time h2 m2) = Time ((h1+h2+((m1+m2)`div`60)) `mod` 12) ((m1 + m2) `mod` 60)

infixr 8 %-
(Time h1 m1) %- (Time h2 m2) = Time ((h1-h2+((m1-m2)`div`60)) `mod` 12) ((m1 - m2) `mod` 60)

-- Examples
main = do
    print $ 7!:30 %+ 0!:30
    print $ 7!:30 %- 7!:31
    print $ 7!:30 %- 7!:30
    print $ 7!:30 %+ 8!:59