Fix parsing and add documentation

src/TaskMachine/Task.hs

@@ -7,7 +7,10 @@
 -- 1. A single line represents a single task
 -- 2. Should be entirely human-readable and easily editable using a simple text viewer/editor
 -- 3. When sorted alphabetically, should yield useful results
--- 4. Completing a task is as simple as changing the @-@ into a @x@
+-- 4. Completing a task is as simple as changing the @"-"@ into @"x"@
+--
+-- [Incomplete task] @"-[ (\<priority\>)][ d\<due date\>][ c\<creation date\>] \<description\>"@
+-- [Complete task] @"x[\<completion date\>][ (\<priority\>)][ d\<due date\>][ c\<creation date\>] \<description\>"@
 
 module TaskMachine.Task
   ( Task(..)
@@ -19,10 +22,14 @@ module TaskMachine.Task
   , Snippet(..)
   -- * Formatting
   , formatTask
+  , formatTasks
   , formatDate
+  , formatDueDate
+  , formatCreationDate
   , formatCompletion
   , formatPriority
   , formatDescription
+  , formatSnippet
   -- * Parsing
   -- ** Utilities
   , Parser
@@ -31,16 +38,16 @@ module TaskMachine.Task
   -- ** Objects
   , pTask
   , pTasks
-  , pDate
   , pCompletion
   , pPriorityChar
   , pPriority
+  , pDate
   , pDueDate
   , pCreationDate
   , pDescription
+  , pSnippet
   ) where
 
-import           Control.Applicative  (liftA2)
 import           Control.Monad
 import qualified Data.List.NonEmpty   as NE
 import           Data.Void
@@ -52,6 +59,7 @@ import           Text.Megaparsec.Char
 
 {- Task -}
 
+-- | A single task
 data Task = Task
   { taskCompleted   :: Completion
   , taskPriority    :: Maybe Priority
@@ -60,33 +68,62 @@ data Task = Task
   , taskDescription :: Description
   } deriving (Show)
 
+-- | Convert a 'Task' to its string representation, which can be parsed by 'pTask'.
+--
+-- If this string representation is parsed using 'pTask', it should yield the original task,
+-- with a few exceptions:
+-- 'taskPriority', 'taskDue' and/or 'taskCreated' might be @Nothing@, but the 'taskDescription'
+-- could include the text version of these in the beginning, i. e. @taskDescription = "(A) hello"@.
+-- In that case, converting the task to a string and back yields a different result.
 formatTask :: Task -> String
 formatTask t
   =  formatCompletion (taskCompleted t) ++ " "
   ++ maybeWithSpace formatPriority (taskPriority t)
-  ++ maybeWithSpace formatDate     (taskDue t)
+  ++ maybeWithSpace formatDueDate (taskDue t)
-  ++ maybeWithSpace formatDate     (taskCreated t)
+  ++ maybeWithSpace formatCreationDate (taskCreated t)
   ++ formatDescription (taskDescription t)
   where
     maybeWithSpace :: (a -> String) -> Maybe a -> String
     maybeWithSpace _ Nothing  = ""
     maybeWithSpace f (Just a) = f a ++ " "
 
---parseTasks :: FilePath -> String -> Either (ParseError Char Void) [Task]
+-- | Convert a list of tasks to its string representation, which can be parsed by 'pTasks'.
---parseTasks = parse pTasks -- That's easy!
+formatTasks :: [Task] -> String
+formatTasks = concatMap ((++"\n") . formatTask)
 
 {- Dates -}
 
+-- | Convert a 'Day' to @YYYY-MM-DD@ format.
+--
+-- Example: @"2018-09-08"@
 formatDate :: Day -> String
 formatDate = show
 
+-- | Convert a 'Day' into the due date string representation, which can be parsed by 'pDueDate'.
+--
+-- Example: @"d2018-09-08"@
+formatDueDate :: Day -> String
+formatDueDate d = 'd' : formatDate d
+
+-- | Convert a 'Day into the creation date string representation, which can be parsed by 'pCreationDate'.
+--
+-- Example: @"c2018-09-08"@
+formatCreationDate :: Day -> String
+formatCreationDate d = 'c' : formatDate d
+
 {- Completion -}
 
+-- | Whether a task has been completed or not.
+--
+-- May include the completion date if the task is complete.
 data Completion
   = Incomplete
   | Complete (Maybe Day)
   deriving (Show)
 
+-- | Convert a 'Completion' to its string representation, which can be parsed by 'pCompletion'.
+--
+-- Examples: @"-"@, @"x"@, @"x2018-09-08"@
 formatCompletion :: Completion -> String
 formatCompletion Incomplete            = "-"
 formatCompletion (Complete Nothing)    = "x"
@@ -94,6 +131,7 @@ formatCompletion (Complete (Just day)) = "x " ++ formatDate day
 
 {- Priority -}
 
+-- | A task's priority, which can be any uppercase character from A to Z.
 data Priority
   = PrioA | PrioB | PrioC | PrioD | PrioE | PrioF | PrioG
   | PrioH | PrioI | PrioJ | PrioK | PrioL | PrioM | PrioN
@@ -101,12 +139,17 @@ data Priority
   | PrioV | PrioW | PrioX | PrioY | PrioZ
   deriving (Bounded, Enum, Eq, Show, Ord)
 
+-- | Convert a 'Priority' to its string representation, which can be parsed by 'pPriority'.
+--
+-- Example: @"(A)"@
 formatPriority :: Priority -> String
 formatPriority p = '(' : priorityToChar p : ")"
 
+-- | Convert a Priority to the corresponding uppercase character from A to Z.
 priorityToChar :: Priority -> Char
 priorityToChar p = toEnum (fromEnum 'A' + fromEnum p)
 
+-- | Convert a character to the corresponding Priority, if possible.
 charToPriority :: Char -> Maybe Priority
 charToPriority c
   | min_value <= value && value <= max_value = Just $ toEnum value
@@ -118,32 +161,47 @@ charToPriority c
 
 {- Description -}
 
+-- | A bunch of snippets that make up the description of a task.
 type Description = [Snippet]
 
+-- | Part of a task's description.
 data Snippet
   = Word String
+  -- ^ A space-delimited word that is not any of the special variants listed below.
   | Space String
+  -- ^ One or more spaces that delimit words.
   | Project String
+  -- ^ A word beginning with @"+"@.
   | Context String
+  -- ^ A word beginning with @"\@"@.
   | KeyValue String String
+  -- ^ A word of the form @key:value@.
+  -- The key and value cannot contain any spaces.
+  -- The key cannot contain any @":"@ characters, but the value can.
   deriving (Eq, Show)
 
+-- | Convert a 'Description' into its string representation, which can be parsed by 'pDescription'.
+--
+-- Example: @"task for +project \@context"@
 formatDescription :: Description -> String
-formatDescription = concatMap toString
+formatDescription = concatMap formatSnippet
-  where
+
-    toString :: Snippet -> String
+-- | Convert a 'Snippet' into its string representation, which can be parsed by 'pSnippet'.
-    toString (Word s)       = s
+formatSnippet :: Snippet -> String
-    toString (Space s)      = s
+formatSnippet (Word s)       = s
-    toString (Project s)    = '+' : s
+formatSnippet (Space s)      = s
-    toString (Context s)    = '@' : s
+formatSnippet (Project s)    = '+' : s
-    toString (KeyValue k v) = k ++ ":" ++ v
+formatSnippet (Context s)    = '@' : s
+formatSnippet (KeyValue k v) = k ++ ":" ++ v
 
 {- Parsing -}
 
+-- | Simple megaparsec parser over 'String's.
 type Parser = Parsec Void String
 
--- Completion
+-- Dates
 
+-- | Parse a date in @YYYY-MM-DD@ format (see 'formatDate').
 pDate :: Parser Day
 pDate = label "date" $ fromGregorian
     <$> integer
@@ -155,11 +213,24 @@ pDate = label "date" $ fromGregorian
     int :: Parser Int
     int = read <$> count 2 digitChar
 
+-- | Parse a date in the due date format (see 'formatDueDate').
+pDueDate :: Parser Day
+pDueDate = label "due date" $ char 'd' *> pDate
+
+-- | Parse a date in the creation date format (see 'formatCreationDate').
+pCreationDate :: Parser Day
+pCreationDate = label "creation date" $ char 'c' *> pDate
+
+-- Completion
+
+-- | Parse a 'Completion' (see 'formatCompletion').
 pCompletion :: Parser Completion
 pCompletion =   Incomplete <$ char '-'
-            <|> char 'x' *> (Complete <$> maybeParse pDate)
+            <|> char 'x' *> (label "completion date" $ Complete <$> maybeParse pDate)
 
 -- Priority
+
+-- | Parse the priority character inside the parentheses (see 'charToPriority').
 pPriorityChar :: Parser Priority
 pPriorityChar = do
   c <- anyChar
@@ -168,16 +239,9 @@ pPriorityChar = do
     Nothing -> failure (Just $ Tokens $ c NE.:| [])
                        (Set.singleton $ Label $ 'p' NE.:| "riority character")
 
+-- | Parse a 'Priority' (see 'formatPriority').
 pPriority :: Parser Priority
-pPriority = char '(' *> pPriorityChar <* char ')'
+pPriority = label "priority" $ char '(' *> pPriorityChar <* char ')'
-
--- Dates
-
-pDueDate :: Parser Day
-pDueDate = char 'd' *> pDate
-
-pCreationDate :: Parser Day
-pCreationDate = char 'c' *> pDate
 
 -- Description
 
@@ -201,30 +265,36 @@ pKeyValue = KeyValue <$> (keyBody <* char ':') <*> wordBody
   where
     keyBody = takeWhile1P (Just "key character")   (not . (`elem` ": \n"))
 
+-- | Parse a 'Description' (see 'formatDescription').
 pDescription :: Parser Description
-pDescription = pSnippet
+pDescription = label "description" snippet
   where
-    pEnd :: Parser Description
+    end :: Parser Description
-    pEnd
+    end = [] <$ (eof <|> void (char '\n')) <|> snippet
-      =   [] <$ (eof <|> void (char '\n'))
+    snippet :: Parser Description
-      <|> pSnippet
+    snippet = (:) <$> pSnippet <*> end
-    pSnippet :: Parser Description
+
+-- | Parse a 'Snippet' (see 'formatSnippet').
+pSnippet :: Parser Snippet
 pSnippet
-      =   try (liftA2 (:) pSpace    pEnd)
+  =   try pSpace
-      <|> try (liftA2 (:) pProject  pEnd)
+  <|> try pProject
-      <|> try (liftA2 (:) pContext  pEnd)
+  <|> try pContext
-      <|> try (liftA2 (:) pKeyValue pEnd)
+  <|> try pKeyValue
-      <|>      liftA2 (:) pWord     pEnd
+  <|>     pWord
-      <?> "description"
 
 -- Task
 
+-- | Parse a parser and a single space afterwards.
 andSpace :: Parser a -> Parser a
 andSpace = (<* char ' ')
 
+-- | If the parser succeeds, return its results warpped in @Just@.
+-- Otherwise, return @Nothing@.
 maybeParse :: Parser a -> Parser (Maybe a)
 maybeParse p = Just <$> try p <|> pure Nothing
 
+-- | Parse a 'Task' (see 'formatTask').
 pTask :: Parser Task
 pTask
   =   Task
@@ -234,5 +304,6 @@ pTask
   <*> maybeParse (andSpace pCreationDate)
   <*> pDescription
 
+-- | Parse a list of 'Task's (see 'formatTasks').
 pTasks :: Parser [Task]
 pTasks = many pTask <* eof