Made code more monadic

src/Pattern.hs

@@ -21,19 +21,14 @@ data Pattern =
   deriving (Show)
 
 match :: String -> DateTime -> Maybe Bool
-match s d =
+match ptrn datetime = parse ptrn >>= \p -> Just (check p datetime)
-  case parse s of
-    Just p  -> Just (check p d)
-    Nothing -> Nothing
 
 parse :: String -> Maybe Pattern
-parse text
+parse text = do
-  | isValid = Just (createPattern $ catMaybes fields)
+  xs <- sequence $ zipWith parseField (words text) constraints
-  | otherwise = Nothing
+  if length xs /= 5
-  where
+    then Nothing
-    fields = zipWith parseField (words text) constraints
+    else Just $
-    isValid = checkFields fields
-    createPattern xs =
          Pattern
            { cminute = head xs
            , chour = xs !! 1
@@ -42,12 +37,6 @@ parse text
            , cweek = xs !! 4
            }
 
-checkFields :: [Maybe Field] -> Bool
-checkFields xs
-  | length xs /= 5 = False
-  | any isNothing xs = False
-  | otherwise = True
-
 constraints :: [Constraint]
 constraints =
   [ Constraint 0 59