Set up basic structure

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