Add Deadline and clean up date expressions

src/TaskMachine/Deadline.hs

@@ -0,0 +1,83 @@
+-- | Tasks and events can have one or multiple deadlines.
+--
+-- This module contains a representation for single deadlines,
+-- and some useful functions for calculating things with them.
+
+module TaskMachine.Deadline
+  ( Deadline(..)
+  , Duration
+  -- * Calculations
+  , relevant
+  , isRelevant
+  , nextDeadlines
+  , relevant'
+  , isRelevant'
+  , nextDeadlines'
+  ) where
+
+import           Data.Maybe
+
+import           Data.Time.Calendar
+
+import qualified TaskMachine.DateExpr as TM
+
+-- | Duration of a task or event in days.
+--
+-- A duration of 1 means one day.
+-- Because of this, 'Duration' values __must always be greater than 0__.
+type Duration = Int
+
+-- | A way to represent a deadline, either as a fixed date or using a formula.
+data Deadline
+  = DFixed Day (Maybe Duration)
+  | DExpression TM.BoolExpr Duration
+
+-- | Find the next date of the 'Deadline' that's important for a certain day.
+-- This returns a 'Just' when the day lies within the duration specified.
+--
+-- If no duration is specified in a 'DFixed' deadline, all days before the deadline,
+-- including the deadline itself, are important (i. e. the duration is infinite).
+relevant :: Day -> Deadline -> Maybe Day
+relevant today (DExpression expr duration) = TM.findNext today duration expr
+relevant today (DFixed day Nothing)
+  | diffDays day today >= 0 = Just day
+  | otherwise               = Nothing
+relevant today (DFixed day (Just duration))
+  | diff >= 0 && diff < toInteger duration = Just day
+  | otherwise                              = Nothing
+  where diff = diffDays day today
+
+-- | A version of 'relevant' modified to take a list of Deadlines.
+relevant' :: Day -> [Deadline] -> Maybe Day
+relevant' today deadlines =
+  let relevants = mapMaybe (relevant today) deadlines
+  in  case relevants of
+        []   -> Nothing
+        days -> Just $ minimum days
+
+
+
+-- | Whether the 'Deadline' is relevant on the current day or not.
+--
+-- This function works like 'relevant', only that the actual date calculated is irrelevant.
+--
+-- @'isRelevant' day = 'isJust' . 'relevant' day@
+isRelevant :: Day -> Deadline -> Bool
+isRelevant day = isJust . relevant day -- Hey, this even reads like English! :D
+
+-- | A version of 'isRelevant' modified to take a list of Deadlines.
+isRelevant' :: Day -> [Deadline] -> Bool
+isRelevant' day = any (isRelevant day)
+
+-- | Calculate all occurrences of this deadline within the duration given.
+nextDeadlines :: Day -> Duration -> Deadline -> [Day]
+nextDeadlines start duration (DFixed day _)
+  | diff >= 0 && diff < toInteger duration = [day]
+  | otherwise                              = []
+  where diff = diffDays day start
+nextDeadlines start duration (DExpression expr _) =
+  TM.findWithin start duration expr
+
+-- | A version of 'nextDeadlines' modified to take a list of Deadlines.
+nextDeadlines' :: Day -> Duration -> [Deadline] -> [Day]
+nextDeadlines' start duration = concatMap (nextDeadlines start duration)