To do List

todo.hs

data User = User { email :: Text }

newtype TodoList = TodoList ([UTCTime, Map TodoId Todo]) 

newtype UserLists = UserLists (Map Email [TodoList])  

newtype TodoId = TodoId Int 

data Todo = Todo 
 { description :: Description,
   status :: Status,
   priority :: Priority,
   dueDate :: UTCTime
   visibility:: Visibility }

data Visibility = Public | Restricted [User]

newtype Description = Description Text

data Status = Done | NotDone

data Priority = Low | Med | High

-- Use a smart constructor to prevent description from being empty.
mkDescription :: Text -> Maybe Description 

create :: [(UTCTime, TodoList)]

update ::  [(UTCTime, TodoList)] -> TodoId -> Maybe Todo -> [(UTCTime, TodoList)]

view :: TodoList -> TodoId -> Maybe Todo

-- This function simply excludes all todos the user is not permissioned to see.
-- Two optional filter parameters are given to the function.
-- If Just UTC Time is passed then the closest TodoList to that time will be picked.
viewTodos :: Maybe UTCTime -> Maybe Email -> UserLists -> UserLists

closestTodoList :: UTCTime -> Email -> UserLists -> (UTCTime, TodoList)

-- Adding feature B:
--   "Ability to go back into the past, and see what a user's todo list looked like at any point in time."

-- Explain how this would work for todo lists that already exist.

-- To handle existing todo lists I would just put them in the singleton list with a timestamp of the current time now.