Set up basic structure

DateExpr.hs

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+module DateExpr
+  ( BoolExpr
+  , IntExpr
+  ) where
+
+import Data.Time.Calendar
+import Data.Time.Calendar.WeekDate
+import Data.Time.Calendar.MonthDay
+import Data.Time.Calendar.Easter
+
+data BoolExpr = BValue Bool
+              | BStatement DateStatement
+              | BLeapYear
+              | BNot BoolExpr
+              | BAnd BoolExpr BoolExpr
+              | BOr BoolExpr BoolExpr
+              | BEq IntExpr IntExpr
+              | BGt IntExpr IntExpr
+              | BLt IntExpr IntExpr
+
+data IntExpr = IValue Integer
+             | IDate SpecialDate
+             | INeg IntExpr
+             | IAdd IntExpr IntExpr
+             | IMul IntExpr IntExpr
+             | IDiv IntExpr IntExpr -- div, not quot!
+             | IMod IntExpr IntExpr -- mod, not rem!
+
+data DateStatement = IsLeapYear
+                   | IsWeekend
+
+data SpecialDate = SJulianDay
+                 | SYear | SMonth | SDay
+                 | SDayOfYear
+                 | SDayOfWeek
+                 | SYearCount
+                 | SMonthCount
+                 | SEaster
+
+evalBoolExpr :: BoolExpr -> Day -> Bool
+evalBoolExpr (BValue v)     _ = v
+evalBoolExpr (BStatement s) d = evalDateStatement s d
+evalBoolExpr (BNot a)       d = not $ evalBoolExpr a d
+evalBoolExpr (BAnd a b)     d = evalBoolExpr a d && evalBoolExpr b d
+evalBoolExpr (BOr a b)      d = evalBoolExpr a d || evalBoolExpr b d
+evalBoolExpr (BEq a b)      d = evalIntExpr a d == evalIntExpr b d
+evalBoolExpr (BGt a b)      d = evalIntExpr a d > evalIntExpr b d
+evalBoolExpr (BLt a b)      d = evalIntExpr a d < evalIntExpr b d
+
+evalIntExpr :: IntExpr -> Day -> Integer
+evalIntExpr (IValue v) _ = v
+evalIntExpr (IDate s)  d = evalSpecialDate s d
+evalIntExpr (INeg a)   d = - evalIntExpr a d
+evalIntExpr (IAdd a b) d = evalIntExpr a d + evalIntExpr b d
+evalIntExpr (IMul a b) d = evalIntExpr a d * evalIntExpr b d
+evalIntExpr (IDiv a b) d = evalIntExpr a d `div` evalIntExpr b d
+evalIntExpr (IMod a b) d = evalIntExpr a d `mod` evalIntExpr b d
+
+evalDateStatement :: DateStatement -> Day -> Bool
+evalDateStatement IsLeapYear d = isLeapYear $ year d
+evalDateStatement IsWeekend  d = weekday d `elem` [6,7]
+
+evalSpecialDate :: SpecialDate -> Day -> Integer
+evalSpecialDate SJulianDay  d = julian d
+evalSpecialDate SYear       d = year d
+evalSpecialDate SMonth      d = month d
+evalSpecialDate SDay        d = day d
+evalSpecialDate SDayOfYear  d = yearday d
+evalSpecialDate SDayOfWeek  d = weekday d
+evalSpecialDate SYearCount  d = ((yearday d - 1) `div` 7) + 1
+evalSpecialDate SMonthCount d = ((day d - 1) `div` 7) + 1
+evalSpecialDate SEaster     d = diffDays d $ orthodoxEaster $ year d
+
+julian :: Day -> Integer
+julian = flip diffDays (fromGregorian 1858 11 17)
+
+year :: Day -> Integer
+year d = let (r,_,_) = toGregorian d in r
+
+month :: Day -> Integer
+month d = let (_,r,_) = toGregorian d in toInteger r
+
+day :: Day -> Integer
+day d = let (_,_,r) = toGregorian d in toInteger r
+
+weekday :: Day -> Integer
+weekday d = let (_,_,r) = toWeekDate d in toInteger r
+
+yearday :: Day -> Integer
+yearday day = let (y,m,d)   = toGregorian day
+                  dayofyear = monthAndDayToDayOfYear (isLeapYear y) m d
+              in  toInteger dayofyear

Task.hs

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+module Task where
+
+import Data.Time.Calendar
+import DateExpressions
+
+data Priority = Must
+              | Should
+              | Can
+  deriving (Eq, Ord, Enum, Show)
+
+type Description = String
+type Amount = Int
+type Duration = Int
+
+data When = Forever
+          | Until Day
+          | During Duration Day
+          | While Duration BoolExpr
+
+data Task = Task Priority Description Amount When