Add uncommitted files

plan.txt

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+normal task
+  - | x(completion date)
+  (priority)
+  (due date)
+  (creation date)
+  description
+
+delayed task
+  (start date)
+  (priority)
+  (due date)
+  (creation date)
+  description
+
+  on update:
+    if after start date:
+      turn into normal task
+
+formula task
+  (when formula)
+  (priority)
+  (due date)
+
+"update":

src/TaskMachine/History.hs

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+-- | An undo-redo history that keeps a copy of every state.
+
+module TaskMachine.History
+  ( History
+  , history
+  , step
+  , current
+  , modify
+  , undo
+  , maybeUndo
+  , redo
+  , maybeRedo
+  ) where
+
+data Step a b = Step a b
+
+-- | Represents the history (only one branch) of some type @a@.
+--
+-- Contains backwards (ctrl+z) as well as forwards (ctrl+y or ctrl+shift+z) history,
+-- as well as the current state.
+data History a b = History [Step a b] a [Step a b]
+
+-- | Create a new 'History' from a single state.
+history :: a -> History a b
+history a = History [] a []
+
+-- | Add a new step to the history.
+--
+-- Any forwards history will be overwritten, as this action starts a new
+-- branch in the history tree.
+step :: a -> b -> History a b -> History a b
+step a (History xs y _) = History (Step y b : xs)  a []
+
+-- | Read the current state of the history.
+current :: History a -> b -> a
+current (History _ a _) = a
+
+{-
+-- | Modify the current state, adding a step in the process.
+--
+-- @'modify' f h = 'step' (f $ 'current' h) h@
+modify :: (a -> a) -> History a -> History a -- not a functor!
+modify f h = step (f $ current h) h
+-}
+
+-- | Jump to the previous state, remembering the future for later redos.
+--
+-- If there is no previous state, do nothing.
+undo :: History a b -> Maybe (History a b, b)
+undo (History (Step x b : xs) y zs) = Just (History xs x (Step y b : zs), b)
+undo _                              = Nothing
+
+{-
+-- | Like 'undo', but returns 'Nothing' if there was no previous state.
+maybeUndo :: History a -> Maybe (History a)
+maybeUndo (History (x:xs) y zs) = Just $ History xs x (y:zs)
+maybeUndo _                     = Nothing
+-}
+
+-- | Jump to the next state, remembering the past for later undos.
+--
+-- If there is no next state, do nothing.
+redo :: History a b -> Maybe (History a b, b)
+redo (History xs y (Step z b : zs)) = Just (History (Step y b : xs) z zs, b)
+redo _                              = Nothing
+
+{-
+-- | Like 'redo', but returns 'Nothing' if there was no next state.
+maybeRedo :: History a -> Maybe (History a)
+maybeRedo (History xs y (z:zs)) = Just $ History (y:xs) z zs
+maybeRedo _                     = Nothing
+-}

test.txt

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+- A simple task
+x A simple completed task
+
+- c2018-12-13 A simple task with creation date
+
+s2018-12-20 d2018-12-24 A task with start date
+-2018-12-20 d2018-12-24 A task with alternate start date